ADR has the advantage especially on agency of more work as it seems there’s a genuine shortage of that (although it would help if it paid more than non ADR).
Also there’s less chance of a major or expensive screwup as theres really not much more for you to do beyond remembering the paperwork and some basic rules. Almost everything else in the case of basic ADR stuff is taken care of by the office.
HIAB has more chance of problems such as people wanting stuff in awkward places or building sites hazards which can be enough of a pain at the best of times. Learning to drive a class C can be stressful enough initially without added stress.
Class 1, some say its a good idea and it does take you out of the bigger pool of drivers. But it does also take a lot more learning and as mentioned it is much more expensive. Honestly not sure if its financially worth it as the pay rate is often very similar.
I’d maybe do class 2 even without other stuff then run that for a year and see if you like it, as long as you can get a job which is the harder part.