Is something like THIS the best dremel tool to use to polish the feed ramp with? Do those tips really stay on the "screw" well enough to work with?

Yes. Those will work fine. They’re $2.99 ea when you buy them individually at Lowe’s or others.  They do stay on.

Yep, that's what I use for mine.  Go easy on any sharp edges...you can round 'em off if you're not careful.

FWIW, in the case of the last CMMG conversion kit that I bought (in autumn 2018), I found that no filing and/or polishing on any surface was required.  It has been plug and play in semi and full (with trip and ABW) auto.  The ramp area was noticeably more smooth and well formed than what was originally on three older conversion units that I did smooth down and contour a bit.
Best of luck with whatever you pursue.
MHO, YMMV, etc. Be powerful; be well.

Why are you having to polish the feed ramp?  Who made it?

Polishing the feed the ramp helps in feeding reliability. 22lr needs a smooth feed ramp and smooth transition into the chamber bore. Mass production doesn't allow for the extra time needed to get these surfaces perfectly smooth.

Some setups run fine without modification needed. Some setups need extra attention to get running reliability. While other people feel that polishing before assembly to avoid any trouble is worth the extra time.

My first 22lr upper I put together would FTF wax coated lead rounds every couple magazines. The round would jam upwards into the top of the chamber and the bolt would jam instead of chambering the round. Polishing the ramp and burnishing the opening of the chamber fixed my feeding issues.

I've built two and bought one.  None required any polishing of the feed ramp.