We have a 775 in a Banebots 256:1 P60 going 10:60 driving an arm, and had problems as well. We had the end of the shaft well supported, but it didn't matter. First the ring gear spun and the output carrier plate was rounding out. So, we removed the roll pins, welded where they were, and filed it flat to match the flats in the ring gear. And we welded the carrier plate to the output shaft. Didn't have any more problems with that. Then the sun gear and planet gears on the last stage stripped, and the ring gear was starting to go. Replaced all that, and it's fine again, but it's only a matter of time. We chose these gearboxes this year because we didn't have the time or money to manufacture our own solution, but, we will not be using these in high reduction or high load situations ever again.
Do you have the technology to machine your own gearbox plates and shafts? If so, you could design a spur gear box using off the shelf gears from AndyMark or McMaster.
Or, you could potentially use AM stackerboxes, if you have the space to fit it. Or maybe a GEM500. You'd probably want an AndyMark Planetary and a Fisher Price or BaneBots 550 in place of your current 775.
Another option may be something like
this driven by a lesser reduction BaneBots P60, or by a drill transmission, such as a Dewalt XRP.
I saw one team using some gears from an inexpensive Harbor Freight winch to drive their arm.
Or, you could go 330 style and use the KOP plastic Fisher Price gearbox, driving two stages of sprocket and chain reduction.