What to do with weaker/unused 4/5* CE?

Honestly, if you're not going to regret it, burn them for prisms

most of those I just burn, like primeval curse and awakened will.

Craft Essences effects are designed to cover almost all buff variations, but sadly most of them have use in ~0,001% of missions (and even in then you can replace them with something else). But sometimes you can never expect when they come in handy (like sure-hit CE's at Nerofest's Cu and Scathach challenge or debuff immunity for NP of Cat, Jeanne and Fran).
What you definitely should NOT burn is:
1) CE's with unique effects like "+50exp".
2) ANY event CE. If event is over just put them away to archive (if your archive space is running low just combine them to LB).

So mostly it's kind of personal thing what to do with the rest. What I do myself is: for 1*-3* I make just one max-LB and use other as EXP, for 4* and 5* if it's useless I just combine 2-5 until LB and put it in archive because it's kinda rare thing and can't just burn it away. Also collection purpure :3

I personally like having at least one of each CE just in case there’s ever a situation I need it

Or, use them as exp bombs too. If you have enough, of course

I just use them for EXP bomb