Hello Richard, it looks like you & I are in a similar position, along with a few of my friends
We have all done long & hard days in the hills, in all weather, but now we are content to do lower level walks in reasonable conditions/3 seasons
One of the problems we have (perhaps that's the wrong word; considerations might be a better choice) is that we have acquired a fair amount of gear, much of which has become reliable, old favourites; it's tried & tested and it all works very well
Interestingly, we have all found that the base weight (gear but no food or water) comes out very similar at around 9 or 10 kg. In that respect you're doing OK
To get this base weight any lower we must accept that we must spend some money on new gear. The problem is that we can't afford to simply splash out on a completely new load of gear, so where's the best place to put it?
You are doing a cost benefit analysis, either mentally and without realising it, or you could even whip out the calculator and do it that way. As you have guessed, you will quickly find out about the Law of Diminishing Returns!
I can see 3 areas where you could save weight -
Tent. I have used an Ultimate Tramp 1, which is similar to the Jetpacker; a Laser Comp 1; and an Akto. But I wanted something a bit bigger, so recently I got an MSR Hubba NX 1; this is excellent, well made & strong. If you can't afford that, look at the Naturehike Cloud range
But be careful when choosing a tent. the makers know that we pay a lot of attention to the weight, and will do all sorts of tricks to confuse us - stripped weight, minimum weight, not including pegs/bags/guylines, or just the bare minimum of pegs. This might not be illegal, but IMO it is definitely sharp practice
The next stage is to look at things like tarps & single skin tents, but they only give useful weight savings if you use trekking poles
You could try using just the flysheet of either of your tents.
Sleeping bag. you say you like your existing bag, but really, you can get a decent bag at less than 1kg. As you are willing to wear clothes inside, this would also help you consider a lighter bag. but shop around for the best prices
Clothing. Your pile jacket is probably too heavy. Have a look at these narrow channelled quilted jackets, in synthetic (do they have a proper name? I'm sure someone will enlighten me)
I use a microfleece top; a Polartec 200 jacket; and a Polartec fleece 200 body warmer
I tried going stoveless a couple of times, but found that although I was OK with cold food, I missed my coffee in the morning, and tea in the evening.
Look at a very small titanium stove, like the Alpkit Kraku, or the various ultralight meths burners
Finally, here's a plug for
www.backpackersclub.co.uk. I've been a member for over 20 years and find them very friendly & easygoing. Many of the members are of a certain age, and have a lot of knowledge on lightweight gear. The club also gets discount at several retailers