How do I trim plywood flush to a frame edge that is bevelled rather than square?

The neatest answer depends on how you cut the bevel in the first place, and it is probably simpler than you fear.

If you cut it on a table saw, tip the blade to the same angle in the opposite direction, set the height so it cuts through the plywood and just skims the frame, flip the assembly and run each edge. The blade follows the bevel because you are cutting parallel to it. Two passes and it is done.

If you cut it with a circular saw and a guide, same idea — set the same angle, clamp a straight edge, and trim.

With a router, a flush trim bit follows the frame with its bearing, and on a bevelled edge that leaves the plywood proud at one face and undercut at the other. Some people accept that and finish by hand; a bevelled trim bit matching the angle does it properly.

The lowest-technology answer, and on a small door it is genuinely competitive: a hand plane.

Plywood edges plane easily if the blade is sharp and you work from the outside in to avoid tearing the outer veneer. Any plane will do — a block plane is fine for this size of job.

The approach that works: rout or saw close, leaving a millimetre or so, and then plane the last bit down to the frame. Working by hand for the final pass means you can follow the bevel by feel, checking with a straightedge across the two faces, and there is no setup at all.

On a job this size I would be finished before I had the router table configured.

One thing worth reconsidering before any of this: whether to trim after glue-up at all.

The alternative is to cut the plywood accurately to size first and glue it in position. That removes the trimming problem entirely and it is how a lot of panel work is done. The cost is that it needs accurate cutting and careful alignment during glue-up, with no margin for the panel shifting as you clamp.

Oversize-and-trim exists precisely because that alignment is hard. So the question is whether you would rather solve an alignment problem before the glue or a trimming problem after it.

For a door with a bevel on the long edges, I would probably still go oversize and trim, because getting a bevelled panel positioned exactly during clamping is worse than trimming it afterwards.

Whichever route you take, mind the outer veneer. Birch plywood has a thin face ply and any trimming operation that lifts or chips it leaves a mark that shows through finish.

Scoring the cut line first, cutting with the good face oriented so the tool exits into the waste, and using a sharp bit or blade all help. On a door, the edges are exactly where people look.