Thanks again for the great responses!
I added more wing area, to start with, on my Mk 2 craft.
I built an Aerospike version. For this I need to use its high TWR to get out of the atmosphere quickly but I can get it into a (regular) orbit with about 550 m/s of dV to spare.
I added even more lift to the Nuke version, but with two engines it just can't accelerate quickly enough once in the upper atmosphere, before the plane falls back down (after which the drag becomes overpowering and it loses speed).
Looking at AeroGav's designs I think I am just using too many Mk2 parts.
Based on this thread I also created a Mk 1 plane that successfully achieves retrograde orbit with enough dV to rendezvous!
It took a long time, partly because I spent days trying to figure out why my efficiency dropped precipitously between two game sessions, only to realise that somehow at some point the cooler had become Mounted BACKWARDS! I then had to figure out how to get it back to Kerbin without exploding...
It may not be pretty, but finally I have a craft that can de-orbit my Kerbals!
Interestingly, I still find the best ascent for the above plane is:
Take off at 10-20 deg.
Increase speed to ~1200 m/s by 10 km.
At 10 km pull up gently (30 deg)
At 20 km engage Nukes
Continue at 30 deg until Alt. reaches 40 km
Level out and push forwards until orbit is achieved
I tried the just fly faster approach, but I get into orbit with much less dV. I am not sure why.
In fact, I find using the jets to throw up the AP while they still can at high speed/low altitude the best ascent for all my planes. I wonder if there's a quirk or mistake I am repeating in all of them, since I can't seem to figure out how to make good use of the high wing area. I've never been able to push the PE high enough while in the atmosphere at all regardless of engine. (Actually, I have trouble getting to any speed that can present temperature problems with the Mk 2 craft - is this a sign that the craft is too draggy to make it?)
Yes I originally started this because I had Kerbals on ships without re-entry pods and it felt super wasteful to send up rockets just to de-orbit them. These planes have taken so much time, and have been frustrating, but once they started to work as Signo says there's so much to explore in different ways to tweak and fly them.