I have the KX3, which I really love, but it is a bit heavy for multi-day trips. Purely from the point of view of weight, the CW only MTR series from LNR Precision seems to be the most popular choice, but as you say in your original post, you need SSB for the time being. One much cheaper alternative to the usual choices (KX3, KX2, FT817) is the mcHF SDR transceiver from M0NKA:
You can buy the boards pre-populated with the SMD components if you are not confident with fine soldering. The cases can be bought off Ebay:
Whilst it does not have 6m and above - neither does the KX2 - this would work out a hell of a lot cheaper, and give a great deal of satisfaction in the building of the kit.
On board batteries may be convenient, but I always use external lithium batteries these days, as you get more control over the voltage and thus the output from the PA. Lithium batteries have way more capacity than internal options. It seems to me that a lot of folks are going down the external battery route these days.
Internal tuners are also very convenient, especially if you are using digi-modes (in particular the JT modes), in which case you can just click to tune if you decide to change bands. On the other hand, end-fed antennas allow you to leave the traditional coax feeder at home and thus save weight that way. I now use a Fuchskreis end-fed antenna, which works fine for me. However, with the mcHF so cheap, you could buy that and still have change for an external ATU like the very popular T1 Auto-tuner from Elecraft:
The mcHF also has a built in pan-adapter, which would cost you another $500 from Elecraft. In fact, you would have plenty of money left over for other important lightweight items, such as a nice light tent:
If I'm teaching grandma to suck eggs here, then please ignore me, but a lightweight tent like the one above could take over 1.5 kg off your shoulders. FWIW, I'm ordering mine (tent) in October.
Just please don't buy the ready made Chinese clone of the mcHF. If we don't support the developers, then innovative products like this will just cease to exist. Will China step in then to keep the innovation going?