>>1188216Your cables are most likely binding, catching, frayed, kinked, or incorrectly routed. Failing that, it could be derailleur hanger alignment like anon said or a sticky pivot or similar in the RD.
Have you adjusted B tension and that the jockey wheel cage is in line with the chainline and not bent?
First port of call in this sort of thing is to check the condition and routing of cables, the derailleur hanger alignment, adjust the gears properly and test ride. If you're in doubt, replacing the cables isn't a difficult or expensive job and rebuilding and regreasing the derailleur (if older) is also worth considering if you're a bit handier.
>>1188242Indexing and chain width (going from 1/8" to 3/32) won't be compatible, and the cost of getting it to work may end up more than just buying a used Ultegra D2 electronic groupset.
Don't do it anon, it sounds like a cool project in theory, but it'll more than likely break your heart and your wallet for no real benefit.
>>1188298Completely rigid, but get some 36mm or bigger tyres and learn how to use pressure for your conditions.
Under normal road conditions and even on unsealed roads, your tyres are enough suspension to get the job done.
Plus, climbing with non-lockable suspension is awful.
