Apologies if it sounds like I’m beating a dead horse here, but I agree with those who say the drivetrain should work fine, and the problem situation as described sounds like results of an incompetent mechanic.

It’s unclear which derailleurs, or if both, are giving the OP some trouble, but I run 8spd Claris out back on the Tern and it shifts fine and runs very silently. I don’t have a front derailleur so cannot comment on Claris in that regard.

It could well be that the shift lever action doesn’t suit her for whatever reason, and so upgrading some drivetrain bits might be a remedy.

The 8spd freehub will take 9 and 10spd road cassettes, so upgrading cassette, rear derailleur and shifters is the basic upgrade option, and a new front derailleur can be thrown in, too.

Beyond that, the rabbit hole is deep, and you could look at changing the freehub to fit 11spd road cassette, as well as changing the crankset or rigging up a mountain cassette with the existing freehub, for example.

I don’t recommend any of that, though, until the stock setup is sorted and shifting right, particularly as someone said if the OP is working with the same mechanic.

I’d put a small bet in on a bent derailleur hanger being culprit here.