a 2008 BF 21 is worth a lot of money....
if we were talking about a 1980 B17 the following would not apply...but...
you may not have this trailer FOREVER....at some point you may want to sell it....modifying it to fit your particular measurements might limit your market when you try to sell (ie. lower price)
given that I would make sure that any mods to address the issues you describe be easily reversible back to stock design/measurements
off the top of my head....the issue of seats being too high could be addressed by building a removable storage compartment that would sit on the floor under the dinette table (only real change would be to shorten the table leg, later you could buy and install a new leg to return to original)....having a slightly higher floor under the dinette table is something some manufacturers use in their original builds.
the issue of the seats being too deep could be addressed by getting thicker foam pieces cut for the seatbacks (even more comfort) and getting them upholstered.....then store the originals seatbacks in a dry location for when you decide to put the trailer back in it's original condition
the issue of the cavernous, difficult to use storage area will never go away...a thicker seatback will even make it slightly worse even...you could learn to live with it or do something drastic: give up on it being interior storage and cut/install an outside access hatch....just like the one under the passenger side dinette seat....you could even source one from bigfoot
if you wanted the exterior to look "perfect"......I've thought of doing something like that on mine but haven't "pulled the trigger" on that one yet...I havew a long list of possible mods for my trailer and that one has not made it to the top of the list
good luck.....where abouts are you in PQ anyways? (I'm originally from the west island of Montreal)...cheers, F
PS. I haven't figured out how to get pictures to appear full size in the posts (you have to click on them to see them full size) I click on the attachment icon to the right of the smily face on the top line of the message box....and follow along after that)