Here's a sentence I think is a bit awkward.
Everyone gets to sing one or two songs, but sometimes our manager will sing for ages by himself until he runs out of steam.
I think this sentence seems like a little awkward, because 1: 'will' is more likely to refer to a situation that is always expected to happen, rather than a situation that will happen in the specific future. So, here I think 'will' doesn't go well with 'sometimes'. And because 2: it would be better to use 'because', rather tan 'but'.
To make this sentence more clear to understand, what if I say like following?
Everyone gets to sing one or two songs, because our manager will always sing for ages by himself until he runs out of steam.
Best Answer
Those two sentences are saying two completely different things:
This one is saying that, occasionally, the manager sings for a long time. It does not actually imply that this always occurs... the word sometimes is telling you that.
This one is saying that the manager always sings for a long time and the because makes the first clause beg for the word only:
If you really want to simplify the sentence and keep the same meaning, the simplest thing to do is to remove the word will:
From what you added in the comments, this may be a good way to word it, with some slight changes for what you're trying to say:
There are many ways to say the last part of the phrase based on what you're actually trying to say. You could say:
Just a couple of options in addition to what's in the example above.