You can use a 9 speed MTB RD with your older 10 speed STI shifters. You need to keep the road FD. The main issue is current 9 speed RD’s are falling by the wayside and except for a Deore here and there, the choices are Alivio and Acera.

A 9 speed XT, or XTR, from the past would be nice with your setup.

Here is my reality. I’m running a 6500 triple with ac 28-38-48 (flat rings but I don’t mind). My RD is an XTR M910. I’m only running an 8 speed cassette, but the largest cog is a 36t. No issues indexing.

If you find an XT 750 SGS it should work fine. Even the 760/770 or 960/970 would be great. But be careful to avoid the rapid rise versions.

John