Is there any special reward if you clear Camelot singularity now?

It will unlock Artoria Lancer,Bedivere,Gawain amd Tristan for summoning in story gacha

U got Bedivere.. That's it

Bedivere. A solid 3* saber. Don't expect it to outperform 4*, but it is one of the best in its class.

you can get the sq from the missions now before the banner goes away
also avoiding spoilers and better farming nodes

You will not be rewarded anything for completing it early if that's what you're asking. You get the same thing now as you would if you completed it later, ie, Bedivere.