there can be a lot of pros to a hackamore, but make sure you select one that is gentle. because it works off leverage, some can be rather harsh. all too often people make the incorrect assumption that because there is nothing in the horse's mouth, it has to be softer than a bit, but that's not always the case.
sounds like you have a good plan in place and are going to still address the bitting issues, while looking for something to offer some relief while in the jumping phases.
another suggestion that may work and may help with bitting is a myler combination bit. my hard to bit OTTB who HATES snaffles or anything jointed, but also tends to lean and pull on a mullen pelham really likes their combo bit. it lets me adjust the nose pressure v. bit action so that i can use very little bit and more nose pressure. my other horse, his full sister, prefers more bit, less nose but will get strong when field hunting so for her i use the same bit but with less nose action so it just serves as a reminder when need be. the mouthpiece itself is limited jointed, so doesn't bother my ultra sensitive OTTB's low soft palate and mouth. they mylers come in a pretty nice combination of mouth - with nose options, so you can choose a very very soft mouth with the nose action and adjust as he gets more used to the bit. not dressage legal, but may give some relief why working towards accepting the bit.