The plan is to get rid of the tablet and put a Amazon echo hub or HA on a tablet that directly controls the aircon in the future
As far as HA integration goes, there's already an adapter available that uses the HTTP API that the tablet exposes. This of course still requires a working tablet though.
The next step for an open solution would be to either write a replacement for the tablet API that talks directly to the control box using RS485 (which will have the benefit of allowing existing integrations to work as-is), or perhaps even ignore the original API and start from scratch with something custom built for HA integration.
The original HTTP API that the tablet exposes is relatively straightforward, but it also commits quite a few sins against HTTP, such as mutating state on GET requests (making CSRF type attacks trivial). This makes a like-for-like replacement a little less palatable in my eyes.
The other tricky thing is that the tablet code is where a lot of the state is kept and the smarts of the system are located. Zone names and config, schedules, temperature sensor pairing, ... replacing the tablet API completely like-for-like might be tricky, but doing just enough to support HA integration (maybe submitting a patch to the existing integration to support the new custom API) is probably a much easier task. There should be no need to rebuild features that HA already has such as scheduling etc.