I wouldn't put the tank at the transom for a couple of reasons...it would require a U turn from the toilet, the distance is marginal without that turn, and venting it out the transom would make aerobic tank maintenance--essential to odor prevention--difficult if not impossible.
I'd like to put the tank forward of the toilet, but that appears to be the galley, which makes it a bad idea.
Where relative to the toilet is the forward tank? I'm hoping it's next to the toilet about where your green line is. If so, there appears to be a similar space in the aft head.
As for upgrading to electric toilets, most macerating electric toilets have the same 6' limitation. The exceptions are the high-end all china "thrones." The best--the Raritan Marine Elegance--is made in the US. Most of the rates of exchange with USD plus shipping costs would make the price too high to be worth it IMO. However there are a couple of decent ones made in Europe. I can recommend one or two when I know where you are. You indicate Sweden, but your command of English and choice of phrasing appears to be American.
--Peggie