Denny's last known session was "LaborDay" on 9/3/82 at Unicorn.
This is a striking example of how far Dennis had fallen at the end. For a man who was driven to record, who had music pouring out of him for years, to go the last 15 (or is it 21?, depending of what dating system you are using) months of his life without entering a recording studio? Really brings home how troubled he was

Big part of that was due to the defunct status of Brian's home studio and the sale of Brother Studio. Dennis had nowhere to record for free, and no money to book studio time himself (hence the "Labor Day" tapes apparently remaining at the studio). The one "Garby"-era recording that we know was done in a professional studio was "Stevie", paid for by CBS out of the BBs' recording budget, and likely intended for the BBs, had they shown interest in finishing it. Since they didn't, it remained unreleased (and they shut down funding of further Brian-Dennis recording activity out of fear - and knowledge - of Dennis supplying Brian with dope). Dennis and Christine did some recording together, and it's a safe bet she footed the bill herself, and likely retains the tapes somewhere.
Man. So sad. I wonder if the band and those around Dennis had any concept at the time of how much the lack of a recording studio left him rudderless. I imagine hindsight is 20/20. Not that it's anybody's fault, but just that it's what he truly needed (amongst other things).
How rare were home recording studios at the time? I guess also, since Dennis had tasted the real deal high-end studio, and knew of the capabilities of using the studio as an instrument (as he did on POB), he'd have found it hard to achieve fulfillment just doing rough demos at makeshift studios.
I suppose that besides Brian's Bedroom tapes (which I think are mostly known and now archived properly, right?), the amount of unreleased, unknown, and hopefully vaulted (somewhere in the world) late-era Dennis tracks must be the real Indiana Jones moment left in the BB saga, that hopefully one day will be pieced together as a puzzle... Hope they aren't decaying too much. It's the Beach Boys' Magnificent Ambersons moment.
Dennis is known to have done some recording at engineer's Tom Murphy's home studio after the sale of Brother, however I believe "Murph's Place", as it was called, was running 16-track at the time - definitely professional-grade, but the standard for some time had been 24-track. A great-sounding record could still be made with 16-track, no question, but I think more to the point was Dennis' increasing lack of focus, along with the fact that Tom Murphy was probably not willing to put up with Dennis' erratic behavior at his home studio any hour of any day, at least after awhile - and who could blame him? Incidentally, there was one Dennis session at the former Brother Studio between the summer '78 sale to Tom Scott, Roy Cicala and company (who changed the name to Crimson Sound), and the 9/82 "Labor Day" session (by which time it had evolved into Unicorn Studios), and that was for the lead vocal on "Love Surrounds Me", 1/2/79 (he redid his lead in the second verse the following day at a Britannia Studios session).