[color=#222222]Your shifters will work with any 9 speed Shimano road or mountain rear derailleur. There “might” be a very slight tweak to the low limit setting on the front derailleur. Probably be fine, but it all depends on the inner cage shape and where the chain is positioned on the 34t. If you eventually look for an older mtb rear derailleur, you’ll want an SGS long cage.]
All very good to hear! I did not think I would have any problem with the shifters but I believe the cable pull will change and need to make sure these old Ultegra "brifters" can handle it! Also VERY good to hear the M592 I have now will work and the 11-34T cassette I ordered will max out it's capacity. But what do you mean by "a tweak"...is that a mod to the derailleur itself or just an adjustment? For an older deraileur I could order a M591, which is another GSG long-cage and a "pre-Shadow" design almost identical to the 592. I can see why an older RD design would work better with older shifters, so I will remember that.
Thank you for all your help, it is much appreciated! I feel confident now my setup with the new cog and derailleur will work.