change Shimano R8050 11-30 rear cassette for an 11-34

Shimano are notoriously pessimistic about things like this, perhaps understandably, but given the R8000 GS will pull an 11-40 i'd be tempted to try your SS with the 11-34 first . Wind that B-tension screw in, or reverse it, and see what clearance you get on the top jockey - i'd hazard a guess that the chain wrap on the SS is OK if it'll work perfectly on an 11-30.

I used a Wolf Tooth 'Roadlink' to extend my medium cage derailleur.  Costs about £25 and saves you a) replacing a derailleur and b) splitting your chain.  I thought that it might be a bit of a bodge, but it works very well.

I'd been running the 32 and the chain together for a few 1000 kms, so replaced my chain anyway.  There are plenty of chain length calculators online to help you with the exact number of links you'll need. 

Depends on the cage length. If it's the longer one, Yes as that's what I have on mine! If it's the shorter one, probably not but might be worth a try if you know someone with one already fitted on a wheel.

11-34 will ( almost certainly) take your drive train out side of the recommended tooth difference and capacity for that rear mech. You'll have to get a hanger extender, and be careful about chain length. You may find it impossible to get a tension setting that allows you to use every ratio available as well.