I get that you are not being dodgy and neither are they. If DH has 2 days spare and wants to do this kind of work, then win/win. Maybe.

I would want to make sure you had it well written out, iykwim, so that if the current people move on, you aren't left high and dry.

We made some rooky tax mistakes and we were treated very well by the ATO. Very helpful and never made us feel like criminals or idiots (we kinda were idiots!). Talk to them if you can. You sound like you will make the right choice in the end, whatever that turns out to be.