I've been in a similar search since I started shooting 5x7 last fall. I love the SP-445 for 4x5 but have had heaps of trouble with developing 5x7 and 8x10 film in the SP-810 (uneven development along the edges), which I'm assuming just comes down to not having found the right agitation technique (or it could be my choice of developer and dilution, Pyrocat-HD 1:1:100). I also purchased the updated version of the Poilot 5x7 reel for Jobo 2500 tanks mentioned in post #12 above, but unfortunately, that gave me uneven rotary development along the spot where the auxiliary film support touches the 5" edges of the film. Again, this could just be an issue of developer choice, and perhaps the reel would be perfectly fine with another B&W developer and/or C41/E6 (the gentleman in the video seems to get unblemished results). The SP-810 tank and the Poilot 5x7 reel are both very well made, and people clearly use them with success, so don't let me put anyone off trying them. They just haven't worked out for my specific application.
During my search, I came across the Zebra Dry Plates Kickstarter to build what amounts to a reimagined version of the SP-445, but for 5x7 and 8x10 sizes. Their Kickstarter ended several months ago, but you can still view it here: .
The guy who runs Zebra, Nejc, is active on their Instagram page, so I contacted him and placed an order for the 5x7 and 8x10 versions. They haven't shipped yet because Zebra is reportedly still in the process of fulfilling orders to Kickstarter backers, but Nejc said he thought new orders for non-backers might start shipping in March. I haven't received mine yet so can't comment on usability. The 5x7 version has a 4-sheet capacity and reportedly uses 790 mL of working solution.
I remember Stearman Press experimented several years ago with producing an 8x10 version of the SP-445 (the SP-2810?) that was intended to also develop 5x7, but they abandoned it because they thought it was too costly to produce and unwieldy to handle. That's what eventually gave birth to the SP-810. The videos I've seen of the large Zebra tanks look pretty impressive, but again, I have no direct experience with them and haven't seen anything in the way of a user report. I'll report back when I receive mine and have had a chance to run some film through them.