Groom's mother etiquette?

If she is this pushy already about what she wants for your wedding I wouldn't involve her any further. I wouldn't accept money to help pay for anything because then it will be what she wants for your wedding instead of what you, and your FH want for your wedding.

If she’s helping to pay for the wedding she gets a say but if not your FH should have a talk with her sooner than later

She is contributing a small amount ($5,000) but my parents are contributing $20,000. I do not want to be rude but is it acceptable to just not keep her in the loop about most things and only tell her big picture things?

Well, no matter what's she's contributing ($5000 is a lot by the way), it would be fair to take her input. Both sets of parents are contributing a bit for us, and so we're including them in planning. If you don't want her input, you don't accept her contribution.

FH should have a conversation with his mother. If she continues being pushy, even after not accepting the gift, and after the conversation, I think it would be to the point where you would refrain from discussing details with her. It would be situation where you would tell her things after they've been finalized.

I totally understand that in the grand scheme of how much your wedding cost, $5000 may not seem like much. But that’s my entire budget. So it’s kind of a big deal for some people.
Anywaaay, I would set boundaries now. Decided what your spending that money on and share those ideas with her. Say catering or flowers. If your choosing between two things, maybe ask her opinion. So she feels like she gets a say, but you were going to pick one or the other anyway.

You don’t have to include her in every decision. Maybe include her for smaller thing that you care less about - invitations? Venue? Not dress shopping! You could also designate some or all of that $5K for the rehearsal dinner and let her plan it.

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I'm sorry, I didn't mean to be rude about the money, I was referring it to being a small amount in the grand scheme of what our budget is.

This is all very good advice. Thank you everyone!