If the chain is falling off even in the 3x configuration, I'd suspect something isn't set up quite right, probably limit screws. Or maybe the chain is a couple links too long, hard to say, but both are relatively simple adjustments to make. Tourney is a lower end spec, but without major overhaul, I still am not sure going 1x is going to transform your ride experience based on my own conversion to 1x8. Part of the XCE issues are probably lack of adjustable rebound, so the suspension compresses then just bucks you back after the initial hit, which is typical of budget coil forks. I guess as long as you're gonna do something, I'd choose the fork over the drivetrain, as long as the Judy has that rebound adjustment (not sure if that's on the $170 one you're looking at, you did say it's air at least). But mostly my main concern is when the bike is new in the ~$600 range, that's a third of the cost of the bike new, probably more like half the worth now. Plus any shop labor to install it if not doing it yourself (cutting the steerer to size, moving the crown race over from the old fork). So if it were me I'd probably just save it for that next bike, but like I said, if you're going to anyway I'd do the fork.
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