Looking to up grade the brakes now as cannondales own don’t seem to smart and are probably a bit tired if the forks were anything to go by.
What would be a good sensible budget option ,have avid ‘s on my other bike which i’m happy with.

If you’ve already got the bleed kit for the avids, some more avids/sram.

If you’re more likely to get them done at a shop, probably which ever shimano you can afford.