Welcome to the forum. Cannot speak much to the tankage issue. My boat has flexible tanks port and starboard just forward of galley and chart table. Expanding this capacity in other areas under berths with additional flexible tanks may be the best option as some of these spaces are irregular. Another possibility is to fiberglass off some sections under berths instead of using the bladder type tanks (flexible).
As to the dingy... I am looking for a Fatty Knees as they row and tow well...not to mention they also have the classic look
that compliments the Frances . Might be able to stow on deck forward of mast....have to measure.
Have you located a boat yet? I started with larger boats ( Cape George 36 and BCC 28 ) The Frances has been the best to suite my sailing , single handed and mostly day sails with occasional cruise.
Best to you