Your question is kind of unanswerable. You're asking if the 6a in four years will be sluggish, but sluggish compared to what? Yes, the 6a will feel generally sluggish compared to most other phones released after it.
A custom ROM matters less than the apps you're running. Are you trying to game, stream, navigate, do-it-all on a four year old phone? It's not going to be a super fun experience compared to a phone released in 2025 or 2026. That much seems obvious.
Does that mean the 6a will be unusable in four years? No - it just depends on how you expect to use your phone in four years. Email/texting/phone will certainly perform just fine.
Look at how photo/video editing capability is progressing on phones. Look at media in general. IG, TikTok.. this sort of thing is a lot more fun on a new-ish phone. It just depends on if you care about that sort of thing.
I plan to keep my 6a for as long as it's kicking, to use it as a home remote for smart devices. I expect that the 6a with GOS will work quite well for such a use case through its life cycle.