Quest on putting om second residence on PPOR

Hi think this is a no brainer but am trying to find if we are missing something on this.

Have 5 acre lot in the NW Sydney with a 30 yr old single level house. Both my boys are young adults and the house is simply too small. We also tend to read and collect and have a huge amount of books magazines etc.

Answer is move and rent, move and buy (looking at around 700-800 ) or build further back on the block .
All up with drive way, 6 car garage, big house we think we’ll eat low 400s.
Rent out the other house, rent from the old will cover 80% of interest on the new.
Never plan to sell, so if we move later we will have two sourses of rent, together with rent from a commerecial strata and super etc.
If the devilupahs offer us the going rate for what is being sold all around us, $4.4M then yep its yours, otherwise we will sit until it happens.

I’m taxable on the rent on the old house etc and I don’t have a question on capital gains, but are we missing something here as it seems to be the logical move.