Okay so I’m working on this right now and removing the mode converter from the extension link changed the derailleur geometry more significantly than I expected (I also changed the angle of the link a little, I don’t think it was actually sitting on the hanger’s limiter nub). It’s more tucked now instead of being so extended, and now I’m wrestling with the “end adjust bolt” — not called B-tension here — which I think was dialed in too far.
In the 11T cog (highest gear) I can now just barely get a finger between the top of the derailleur and the chainstay, which is apparently pretty close to how it’s intended to be: significantly more tucked than modern trail derailleurs.
With these changes I’ve gone from thinking my chain was too short to thinking it might actually be a little too long (for use without a tensioner, anyway). Am considering swapping the MRP G5 SL on there before making any changes to the chain length, so I can see if this turns out to be just right with a tensioner added…