First - I've only been able to activate 7 Summits (some activations were the same Summit at different times), but had a great time on them all. I have the 817 (non "ND" model), and have had it quite a few years before I found out about SOTA. I did drool over the KX3 and then the KX2 when they came out, but decided to upgrade the 817 instead of paying significantly more for a new radio. Not knocking the KX2/KX3, I'd love to have either one. I've just gotten used to using the 817 on HF, VHF and UHF. Also use it for satellites on VHF/UHF.
I sent the radio to W4RT and had the DSP, dual filter board and upgraded battery system installed. Since I also like using the 817 mobile, the DSP makes a big difference on the road (signal reports are about as good as with my Icom 7000 when using the Hustler Mag Mount/2 foot mast/band resonators on the truck). And even at a campsite the 500 Hz CW filter makes a difference. I mostly use the 817 with an approx 7Ah LiFePO4 battery, with the internals used for really quick setup. The mods cost a bit, but not near as much as a new radio. And the company was very good to work with. So you do have some upgrade options to make the radio's capabilities a bit better. I didn't do any "screwdriver tuneups" or "maintenance menu adjustments" on my radio. And it's worked fine for at least 15 years. You can use the "Widebander" program (google it) to get 60m on your pre-ND (and some early ND I believe) radios. It's a software change, so no danger of a soldering iron slipping. And the change can be reversed by the same software. Haven't tried this with my radio, have plenty bands to play with.
Along with the 817 I mostly use the LNR 10-20-40 Trail Friendly antenna. Easier/quicker to set up than a dipole (requires less supports), and is resonant, so no tuner required. Was testing this antenna a few weeks ago with a new 20 foot telescoping fishing pole (squid pole/crappie pole) at a local park here in South Carolina and heard NS1TA calling CQ SOTA on CW. Turned out he was on Mt Katadhin in Maine, at the north end of the Appalachian Trail. Was a scramble to dig out the paddles and set up the radio, but fun to make chaser QSOs on both CW and SSB. The radio is flexible and easy enough to use that even this quick QSO could happen. A few menus, but most can be pre set. And those that need to be changed often are easily reached. There are also quick reference guides available for the radio that make operation a lot easier/quicker.
Have never used a KX3/KX2, but have heard lots of great things about them. Just decided to stick with the 817 since I was used to it, for cost and since I do use the included VHF/UHF a bit. Overall it's been a great radio for me, and have had a lot of fun with it. It is a little heavy, but not bad in a backpack. Especially with the new LiFe batteries that weigh about a third as much as the SLAs.
Also have an LNR MTR radio (20, 30 and 40 meters), but still getting used to it. Also fun to use, just have to get my nerve up enough to do an activation on CW. With this one, 8xAA lithium batteries, a small paddle, earphones and a resonant wire antenna you'd really have an effective lightweight setup for a long hike.
Best of luck whichever radio you choose, and have fun!