Il ne sera plus jamais qu’un ami. would rather translate to “[From now on] he won’t be only a friend”.

“Only ever” would have two distinct translations in your examples (for two different meanings) :

• “I only ever get to see him during the holidays”

Je ne le vois que pendant les vacances

• “He’ll only ever be a friend (and nothing more)”

Il ne sera jamais qu’un ami (et rien de plus)

Although the second one can have a very ambiguous meaning.

For instance, if you argue with your wife about her coworker and she wants to reassure you, she will say “Il ne sera jamais qu’un ami !” with a tonic tone : she will emphasize on the fact the the coworker will never be nothing more.

If she says “Il ne sera jamais qu’un ami …” on a sympathetic tone, she will implicitly say that she will always consider him to be more than a friend.

Your first attempt is almost correct, you need to move que after the object of the verb (and du is not correct here btw).

Je n’ai jamais l’opportunité de le voir que pendant les vacances.

The second one is a little trickier because plus can mean two things, and plus jamais means never again (not never more). The second one should be:

Il ne sera jamais qu’un ami. (never more is implied, just like in English)

Or alternatively:

Il ne sera jamais plus qu’un ami (here jamais means never: "He’ll never be more than a friend".)

(I used bold because the s is not silent)

The thing is, jamais in the sense of ever is rare and very literary, it’s almost never used in spoken French. So it’s correct for an essay (although your teacher might tell you the same thing) but I wouldn’t use it for colloquial language.

For the first sentence you can juste drop it:

Je ne peux le voir que pendant les vancances

Je n’ai l’occasion de le voir que pendant les vacances