If you don't own the SLX rear mech already just buy the GRX one, if you need to get it to work with bigger cassettes you can always add a Roadlink, much neater than a TanPan solution
Don't have anything at the minute, just saw that the SLX was £50 new as opposed to the 11 speed GRX being a lot more and I vaguely remembered hearing that GRX was compatible with MTB bits.
If you don't own the SLX rear mech already just buy the GRX one, if you need to get it to work with bigger cassettes you can always add a Roadlink, much neater than a TanPan solution