cutting plywood is a major challenge for workshops that want to use as few power tools as possible

The answers in this StackExchange post are useful and illuminating. Their main conclusion is "plywood is a modern material that requires modern tools to cut it cleanly". However, there are some good detailed tips there for making it work with hand tools.

a circular saw

The option that would make these cuts easiest would be to buy this small DeWalt circular saw, and a carbide-tipped blade. But that is one extra power tool that I don't necessarily have the space for.

table saw

The vast majority of people use either a table saw or circular saw to cut plywood. Cutting it any other way is unusual. But I don't have space for a table saw.

jigsaw

There are various bits of advice here and here on which blades and techniques to use.

Specific recommendations for making clean cuts:

• Disable pendulum motion.
• Blades to use:   A traditional "clean cut" wood blade, or the Diablo Bi-Metal Jig Saw Blade for Wood (with bi-directional cutting).
• Use gaffer tape to cover the top side of each cut. If you're using a bi-directional blade, then tape both sides of the board along each cut. If you don't have gaffer tape, or the cut doesn't justify the expense, then use masking tape (though its decreased adhesion may allow a few fibers to lift).
• Use your anti-splinter insert, if your jigsaw has one. (mine does)

a rough cut, followed by a trim router to finish to the line

As suggested here.

Although this would also require purchase of a new electric tool, I think I already need to buy that for a separate task — cutting T-slots in wood.