Questions after a test-run grout replacement
I used an oscillating multi-tool with a carbide circular blade to grind out the grout.  I took it all out, and used a chisel to removed whatever remained. Re-grouting is a grueling task; I could not find anyone to do it.

Sounds like you need to thin that grout.  Maybe some latex additive would help. Grout should be pasty but flowing.

When all else fails, I use tongue blades to smooth caulk and grout.

The right consistency grout will not leave bubbles and voids. It will flow and be heavy enough to sink to the bottom.