Re: Recommend for trailer sailer with a real head?
That 6.9 is a nice-looking boat. One thing to watch out for re: composters is their height. They are ~5-7" taller than typical heads or PPs, tho roughly similar in footprint. We also are leaning composter, but one reason we are having to move it aft is there isn't enuf headroom forward (on a 30'er!). I highly advise building a mockup before plonking $1k on an airhead. I nested two 5gal buckets & added a hinged lid the size of the Airhead's. That gives us the 19" height of the real thing & lets us sit on it in various places. Right by the companionway, where headroom is 6', the mockup barely fits.
Same for the OP: research marine toilets you might want, knock together a model (including room for the pump handle) & see if you could fit it on prospective trailer sailers. Then find a box the size of a 7 gallon holding tank, & try to site that. You'll quickly see the virtues of a plumbed portajohn. Thetford and SeaLand make some nice ones, even models with electric flush. Far cheaper than a full marine head system, too.
Albin Ballad 30, Fionn