belt sander
A belt sander is certainly one way this could be done and it's one of the methods I was just about to suggest1 after reading the first part of your Question.
I'm somewhat concerned that this won't be even enough though. A more geometrically precise method would be better, I think.
If the belt sander is fixed in place, has a hard platen and the pivot point doesn't shift there's no reason this couldn't be accurate enough. Circle-shaping is now often done via sanding, using jigs that allow the workpiece to rotate against a disk or belt sander.
Tip: start coarse! You don't want to try to remove this much wood with a 100-grit belt, for bulk material removal you want to begin at 60 or even coarser and work up from there. I know some users would start at 36 grit for something like this as extreme as that sounds (if doing this entirely by sanding).
My other thought is to adapt the jig so that it can hold a jigsaw at an angle and trim the edge that way. It seems tricky, though, and I don't know if I trust my jigsaw enough
You could remove the majority of the waste with your jigsaw. You'd want to cut well on the waste side of the line because of the expected wobble in the cut at certain areas along the perimeter (as the grain orientation relative to the cut changes).
But even if you could get the cut really neat you should expect to have to refine the sawn surface a lot afterwards. This doesn't argue against using the jigsaw — you'd have a very similar amount of cleanup to do if the cut were done on a bandsaw, which is the saw many would recommend most highly for this cut.
Hand sanding to finish
Regardless of whether you do this entirely by sanding or not, expect to have to do a fair amount of hand sanding to complete the job. Use a block to back the paper2. The two portions of the circle that are towards to ends of the boards will be tougher to sand than the sides as end grain is always harder than long-grain surfaces, so you'll need to concentrate more effort there.
In general there's no need to sand beyond about 180-220 grit, but for the end grain it's worth taking the time to go up higher, to 400 grit or a bit finer. End grain sanded to higher grits looks better anyway but it finishes better too (helping to avoid the usual tendency it has for going much darker than the rest of the wood when finish is applied).
Remember to slightly soften the top arris. Leaving it sharp would make the table surprisingly uncomfortable or even hazardous to sit at, press against or bump into3, plus finishes can creep away from sharp edges, so it's necessary to round them over slightly. I find a quick sand, just 2-3 swipes, with very worn 250 is often enough for arrises to stay looking sharp but be comfortable to handle. But you can give the edge a definite rounding if you prefer, it's entirely a matter of personal taste.
1 My main suggestion was going to be to do it using hand tools but I'm presuming you don't want to go there so I'm leaving that out :-)
2 If you don't own one make one now from a piece of scrap. You may find, as many have before you, that this 'temporary' sanding block will stick around in the workshop for many years! I still have the first sanding block I made, it's just a length of 1x2 with cork glued to one face that I had no expectation would last more than a couple of months and it's now over six years old.
3 I've cut myself a couple of times on 90° edges in harder woods like oak and this angle you're creating is more acute than that.
