Large hole in plywood

A router with some sort of simple guide will do this easily and neatly. I made myself a simple guide for my (very cheap) Ferm router for exactly this job, it worked very well.

A jigsaw will sort-of do this, but even in a firm guide, it'll tend to wander.

Several ways, depending on what tools you have. Jigsaw out 170mm hole. Finish with a sanding drum in a drill. A router with a bearing guided cutter, and a circular template of the appropriate diameter (probably something like 200mm). You just run the bearing along the template, and there you are.

A beam guide trammel is the simplest way. That typically just needs you to fit one of the fence mounting bars and attach a spike to the end of it. You then use the thing like a large pair of compasses.

Thus:

This is what I did to make several rings in 18mm plywood. I made a jig out of a rectangular piece of plywood screwed to the base of the router with a hole cut where the bit goes. The pivot point was just a hole with a screw screwed into the workpiece. Iirc the bit I used was just a 6 or 8mm straight cutter. It was all a bit of a bodge but the rings produced were perfect.

Sam