Ridley is pretty iconic but he has never really been an endgame boss. I think having him be post lunatic pre moonlord makes sense and then post moonlord should be reserved for an enhanced version of Ridley and Mother Brain.
Ridley has several varations in the games, actually. In canonical order they are: Meta Ridley, Omega Ridley, Proteus Ridley, and Neo Ridley. Meta Ridley is basically a resurrected cyborg version of him, Omega Ridley is Meta Ridley infused with Phazon (probably the most difficult), Proteus Ridley is him resurrected
again but with fewer cybernetic parts, and Neo Ridley is a mutated clone of Ridley created from X-Parasites. The one that would probably make the most sense to use is probably Meta Ridley, as Neo Ridley, while it is the most distinct one, probably shouldn't appear unless there's already a theme of X-Parasites happening as that's a
super major threat that can't just go unnoticed, and Omega and Proteus Ridley are less known than those two. (I might be getting some reasoning wrong here, I didn't play the prime games, so Omega Ridley probably is in the exact same boat as Neo for all I know.)
I don't really know of many final bosses of Metroid games but the Mother Brain fight is iconic (both the NES version and the SNES version) and makes sense as a post moonlord boss IMO. At least the final form of mother brain. The initial lab fight could be pre-moonlord and the loot from it + moonlord + enhanced ridley loot could be part of the summoning mechanic for the final version of mother brain.
From final bosses to choose from (in canonical order), there are:
Mother Brain, Mecha Ridley (just a robotic imitation of Ridley, which is why I didn't mention it earlier), Metroid Prime, Dark Samus, Aurora Unit 313, Metroid Queen, Proteus Ridley, and SA-X. (In the three games that shall not be named there are
Master Brain, Gorea, and Phantoon, but... yeah let's not please.) That's a pretty good selection to choose from, I think. Mother Brain is definitely the most iconic but many of the other ones are rather infamous, so I don't know which would be best to pick.