“Elles” refers to the tax exemption suppression and the stricter definition of the farmers status.

“Mesures” means “actions”. The “suppression” and the “stricter definition” are actions.

Quant à lui is an adverb phrase that signals that the clause applies specifically to the entity that the pronoun lui refers to. The pronoun agrees with its referent, so the phrase can be quant à moi, quant à toi, quant à elle, etc. The referent is usually the subject of the clause, but can occasionally be a complement of the verb.

Here quant à elles refers to the subject, which is “la suppression des exemptions fiscales dont bénéficient les agriculteurs et une définition plus stricte de leur statut”. The use of quant à elles indicates that these particular measures are sensible, without implying whether other measures that the text might be mentioning are or are not sensible.

The sentence could also have been written

Quant à la suppression des exemptions fiscales dont bénéficient les agriculteurs et une définition plus stricte de leur statut, elles semblent être des mesures de bons sens […]

Starting with the subject feels a bit more neutral here. Starting with “quant à …” would highlight some form of opposition or complementarity between this sentence and preceding one.

The original sentence should have been “… semblent quant à elles être des mesures …”, but having the phrase quant à elles allows “être” to be dropped without seeming too unnatural. Normally “sembler” is followed by an adjective or an adjective phrase, not a noun phrase (sembler is analogous to the English “look” or “seem”, sembler être followed by a noun to “look like” or “seem to be”).