A tungsten carbide burr might do the trick.
I have a set for use with my Dremel and have found them very useful.
Better on stainless steel that HSS bits in my experience.
There's a set here which might have one or two in a suitable profile that might suffice:
https://www.ebay.co.uk/i/31287593895...b5054c5b135a6a
Hope that might help.
Every success in your efforts.