That last one isn't a seat though, as far as I can see. I have similar issues and I've looked at these sorts of things. I think I was more into the shooting stick idea, which is something you stick your bum on and lean on, like this
https://www.mobilitysmart.co.uk/economy ... 541e007961
Or maybe just some sort of folding chair to carry. There are 2 main types:
1. The kind of chair you get at festivals that fold down in a couple of seconds, but are a bit bulky. I have one of these. It's not bad. Digs in the thighs a tiny bit where a leg is attached. Weighs about 2kg and is about 2.5 ft long x about 6" dia when collapsed, from memory. They come with a bag with a strap you might be able to put over your shoulder. It would go in a LARGE rucksack, sticking out the top probably. I have carried it on a bike like that.
2. The ultra-light, compact ones backpackers buy. These are more expensive, take longer to set up and take down, and may or may not be comfortable. You are closer to the ground. May be hard to get in and out of. These vary quite a lot in design, weight, cost etc.