In the end though, after buying presensitized PCBs and doing a few test etchings, I decided not to do this at home anyway. I simply have too few time slots long enough to do the setup, etching and cleanup.
What I ended up doing instead was redoing the boards and send them off to be professionally produced. I have shrunk all the boards as much as possible (while still using through hole components). I've split the internal excitation board into two separate pcbs so that all pots are soldered to a board.
In addition, I've combined the LP/BP/HP versions of the voice board into a single circuit. This means that you have to add some solder bridges and that some parts are left out for some of the boards, so populating them is slightly harder. However, it also means that I was able to make dual channel boards that, when repeated 7 times, turns into the full voice board. Using a service like dirtyPCBs, 7 similar smaller cards are much cheaper than one large one.
In retrospect I've realized that I could possibly simplify the circuits a bit, since the left channel on a card will never be HP and the right never LP (or the other way around, I can't remember).
I've also managed to jam all the non voice boards (except one of the internal excitation parts) into two 10 x 10 cm pcbs, keeping the cost really low.