Actually, in most of the sentences you gave, the words are already indirect pronouns:

• Je me bats contre lui.
• Je compte sur toi.
• Il est bon envers moi.
• Il est bon avec moi.

Only in your second sentence there is an indirect noun, and no indirect pronoun:

• Je crois en Dieu

Also, in your first sentence, please note there are both direct and indirect elements:

• "Il fait le gâteau pour Élise" : "le gâteau" is direct, and "pour Élise" is indirect
• Il lui fait le gâteau. : "le gâteau" is direct, and "lui" is indirect

Please note also the detailed meaning of "lui" depends on the verb, for instance:

• " Il lui fait le gâteau " means " Il fait le gâteau pour elle "
• " Il lui lave la figure " means " Il lave la figure à elle " (and not " pour elle ").

The verb and the context will give the proper meaning.

All these pronouns (and the noun) are COI.

There may exist the possibility you are looking for, at times, however, you must change the verb and sometimes also the adjective, and the pronoun is not necessarily a COI; it can be a COD. (Your second instance (Je crois en Dieu.) is not workable; I replaced it with "Je crois en lui.".)

• Je me bats contre lui. → Je le combats. (COD)
• Je crois en lui. → (It seems there is no possibility.)
• Je compte sur toi. → Je te fais confiance. (COI)
• Il est bon envers moi. → Il me traite avec bonté. (COD) — Il m‘est bienveillant. (COI)
• Il est bon avec moi. → (as for the preceding item)

Addition prompted by user silph’s‘s comment

The sentence in which you use the word "Dieu" has nothing to do with the one I substituted for it: you cannot have a pronoun that stands for "God" or anything for that matter and that you could use anywhere, unless you specialize it to that individual as you might do for God in referring to God as "He" (capital letter). Your confusion is due to the fact that you believe that there is a correspondence between constructions with a preposition and constructions with a COI pronoun. This is not so, at least not as a rule. Only sometimes is it possible. It is a matter of usage. As the language was created, sometimes the possibility was included in the language and at other times (that is for other verbs) it simply was never implemented and much less used.

You do have this perfect grammatical correspondence, for instance, for the verbal form "faire confiance à", yet you do not have a perfect correspondence because what you are saying is not exactly the same in both sentences.

• Je fais confiance à toi. → Je te fais confiance.

In the first sentence you are really saying "I trust you and really nobody else."; you single out the person. In the second sentence this nuance is not apparent at all. So, I hope you are getting a little idea of what usage makes of perfect correspondences.

"Indirect object" is a somewhat fuzzy category (inherent argument of a verb that’s introduced, in at least its nominal form, by a preposition) that covers a rather large number of phrases not always united in their morphosyntactic behaviour, and that seems to be what’s causing your issue here.

Lui/leur is the pronoun corresponding to some indirect objects, but not all of them.

More precisely, lui/leur, as a direct descendant of the dative case in Latin, roughly continues its uses in Modern French, that is to say:

• it refers to the recipient of an action (je lui parle, je leur donne une pomme, je leur envoie un SMS)
• it refers to the person affected physically or mentally by an action, in good or bad (il lui a mordu l’épaule, je leur ai préparé un bon petit plat, ça lui arrive)
• it’s used for a set of less common uses in specific constructions, including possession (c’est à lui), obligation (c’est à moi de le faire, je lui ai ordonné/demandé de le faire) and the so called ethical dative (almost never used in the third person, but see je vais me lui parler, where me is a supplementary ethical dative pronoun)
• it’s the default third argument of a verb. Most other prepositions are semantically motivated, but arguments marked by the preposition à and pronominalised by lui/leur can appear totally absent of reason when the subject and direct object arguments of a verb are already filled. You can this most easily in the causative construction where a displaced subject can either be marked semantically as an agent by the preposition par or fill one of the empty default object slots of the verb: Marc lit -> je faire lire Marc/à Marc/par Marc.

In all of those uses, the dative pronouns correspond to noun phrases marked by the preposition à. In one of those uses, it overlaps with the (semantically motivated) use of the preposition pour to mark beneficiaries of an action:

Je prépare un bon petit plat à mes enfants > je leur prépare un bon petit plat

Je prépare un bon petit plat pour mes enfants > je prépare un bon petit plat pour eux

But as you can see from the correspondence above, this semantic overlap doesn’t mean that there is a syntactic link between "je leur prépare un plat" and "je prépare un plat pour mes enfants". Presenting it as the pronoun leur replacing the noun phrase "pour mes enfants" is simply wrong. They’re two different sentences that mean almost the same thing.

When pour isn’t used to mark the beneficiary of purely good willed action, this correspondence completely breaks down:

"Je travaille pour Mohammed" -> "Je travaille pour lui" but never *"Je lui travaille"

"J’ai voté pour Hélène" -> "J’ai voté pour elle" but never "je lui ai voté"

However, I would be remiss if I didn’t mention the one case where there truly is a direct correspondence between indirect objects not marked by à and dative pronouns. A limited set of verbs whose indirect object refers to human beings strongly (and generally negatively) affected by the action of the verb can be pronominalised by lui/leur (or their first and second person counterparts):

il court après ma sœur -> il court après elle OR il lui court après

Il crie souvent sur ses enfants -> il crie souvent sur eux OR il leur crie souvent dessus

In neither case can you say "il court après à ma sœur" or "il crie à ses enfants souvent" without wholly changing the meaning of the sentence. There is no set of two semantically overlapping constructions here, but indeed a novel usage of the dative pronouns.

Note that this use is further restricted to the so-called "orphan prepositions", i.e. those who can be used alone (in their usual form or a strong adverbial one like dans-> dedans).

These two conditions eliminate the possibility of using dative pronouns with all of the sentences in the question, but "je me bats contre lui" since those verbs are all fairly positive and don’t strongly affect the direct object. Furthermore, envers and en can’t be orphaned. In fact, most occurences of this construction involve the preposition sur/dessus, that often marks the target of a violent action (crier dessus, taper dessus, tirer dessus, etc).

A third constraint is that this is only possible if the verb doesn’t already have a pronoun that fills the same slot in the template of the French verb. In other word, because se and lui can’t cooccur, this makes sentences such as "il se bat avec lui" or "elle s’enerve sur lui" impossible to turn into *"il se lui bats avec" or *"elle se lui énerve dessus", despite fitting the semantic requirement and using prepositions that can be orphaned (indeed, you can instead say "il se bat avec" and "elle s’enerve dessus"). This eliminates the possibility of this construction to be used with "Je me bats contre lui", since me and lui cannot be used together on the same verb (outside of the aforementioned use of the ethical dative)