Evolving Beldum

You have the candy to evolve him, but you don't have the candy to power him up. Wait until you have enough candy to evolve and power to your desired level, and then evolve your best one.

I'm in a similar position with Bagon, with enough candy but my highest IV is also 'only' 91. I'm going to wait it out a little bit and hope for one that's 96+. I'd suggest you try to wait too! At the rate you hatch eggs you stand a decent enough chance of obtaining a better one. Or, you know, Niantic could decide to switch Metagross' moves up at some stage and you could save some TMs trying to get away from BP/FC.
I don't think Metagross is so important in the current meta (heh) that you would desperately need one asap - yes, he's really good for Jynx raids, but Jynx has a bunch of other, more obtainable, viable counters.

Sure, you could hatch a 96% soon, or even find a weather boosted 100% one tomorrow. Or maybe you spend the next six months waiting for a half decent one.
Nobody know.

And Metagross isn't a great Pokemon (it's good, but there are better ones), so will you really use it much?

I say evolve it now. 91% is good and you'll still have 26 candies left (I hope my math is correct). Hatch a few, add some rare candies and you'll have enough for another evolve if you get an even better one.

No! If the next next (...) next community day features Beldom...

"and am pretty impatient"

Then evolve it, no worries and that's not a bad one either

Here's my tale. I caught a 15/10/12 (84%) wild weather-boosted Beldum on Friday who was at 7000 dust (whatever high level that is). I was using my one lucky egg to evolve new hoenn pokemon in the last few hours of the double dust event, so I evolved this beldum into a 3200 or so CP Metagross. It even came out with double psychic moves, great!

Today I hatched a 98% 15/15/14 beldum. Don't regret what I did. I'll eventually evolve this one (barring the 100% coming along).

If you evolve it now, the next 10km egg you get will be a 93%+ Beldum.

If you wait to get that 93%+ Beldum, you won't get it anytime soon, if ever; that 91% will be the best you come across.

Let's put it this way - I hatched a 98% Bagon 14 15 15 , yet I'm not going to evolve it until I have enough candy to evolve and max it at the same time ... because you never know if you will catch a better one before you have all the resources required.