Quality plywood is strong. A lot stronger than cedar strip. The weak point is at the joins, but even those areas are strong, the corners adding to overall stiffness in the construction, especially if they are filleted.
IMO 9oz tape at the seams is overkill. We cover strip hulls with 4 and 6oz cloth, and join strip decks and hulls with the same cloth, or tape. There is no need to use heavy tape on the corners, except perhaps for abrasion resistance on the keel, but then you would be better off with some KeelEasy.
For normal sea kayaking 3mm or 4mm, with tape on the corners and nowhere else, would serve most kayakers just fine. If you are really rough on your boats, go for the 4mm. The main area to watch is the deck behind the cockpit, but only if you have a small cockpit and need to sit on it getting in and out. A layer of 6oz cloth under about two ft of the rear deck behind the cockpit will add a fair bit of stiffness there.
Scratches in unglassed ply is easier to repair than on glassed boats.
The one caution I would make, is that because you won't have the added waterproofing of the thick epoxy/glass composite on most of the boat, it is good to ensure good epoxy penetration of the first coat. (the saturation coat). This is easily done, by brushing or rolling on The first epoxy coat fairly generously, and giving it a good blast with the heat gun as you go along. This will make the epoxy very fluid and, at the same time force expanding air out of the timber, allowing the first coat of epoxy to penetrate well into the surface. A couple of overcoats on top of that and you are set for life. Don't forget to varnish or paint to finish off.
People were building boats in ply long before glass and epoxy came along. I built a ply on frame speedboat that had a 6mm hull, no glass, and it took an pounding for years in our short choppy waters. This was before the days of epoxy saturation though, so it eventually rotted.
My two cents.