Hi @dmuensterer
Did you try the fix that worked for the issue you linked? Ziti identies are failing after upgrade of edge tunneler - #15 by qrkourier
As far as upgrading, most of the work in 1.2.0 -> 1.5.4 has been for HA and OIDC support. If you don't use either of those features, then I'd update to the latest 1.1.x release. Note that HA is still in beta, but should be functional unless you're using posture checks. Finishing the posture checks work for HA is the last blocker before releasing a release candidate for HA.
For tunneler/server compatibility, they should be compatible unless you're using HA. The HA support in the tunnelers has been evolving and there are cases where HA enabled tunnelers won't work well with certain versions of the routers/controllers.
There is some performance work coming in 1.6/1.7 that may be of interest, but will also require changes to the tunnelers. Something to keep an eye out for.
Paul