Can easily get 20-30W through a TO-220 and thermal pad, up to maybe 50W with thin, conductive pad and spring clamp. Even with greased contact (live heatsink?), I would be very cautious about more, despite what the datasheet might say (that's a rant for another thread however).
The problem is 1. yeah you've got good sized copper pours, but they're all thin, and 2. most of the meat of the board is FR-4, which is terrifically bad thru-plane, it's doing almost nothing. Obviously you can address these to some extent by fab (more via cross section, heavier copper), but that's pushing into "heroic" territory and one wonders if it would've been cheaper to use the bolted package in the first place?
You can run some guesstimates based on via barrel thickness, copper conductivity, board thickness (you might opt for 1.0 or 0.8mm stock, incidentally), and filling (solder or whatever as applicable). If the estimate is questionable give or take a factor of 2, you may want to order a quick proto and test it (e.g. with a transistor or resistor).
Solder -- typical result is, because it has such worse conductivity compared to copper, even a solid solder-plugged via is about half the Rth of an empty one. Not nearly as amazing as it maybe looks like it should be. But also nothing to sneeze at!
Incidentally, pure tin is better than pure lead, and their alloy is worse than either; lead-free is actually beneficial here.
Or you can have it both ways, SMT and THT -- a TO-220 can be mounted over a hole in the board, so it can mount to the heatsink nearly flush with the board while the leads are soldered in at a modest distance (or even SMT'd against the board, but I'd maybe recommend not doing that at mains voltage). Obviously the heatsink will need some clearance to the thru-pin tails, if applicable.
Tim