I used to use (now KX3) the Elecraft T1 ATU with my 817 and an end-fed wire antenna. It’s great little compact tuner that you can velcro to the top of your 817. It does have a 20w max thou.
50 W radios are not very common. I decided to build an amp to my KX3 to take it with me on shorter tours. The amp is similar to the FT817 Amp Projekt of VK3YY. I made it to operate on 20m, 30m and 40m. Since I use a linked dipole a tuner is not needed. With a 4.5 Ah LiFePo4 Battery I operate up to 2 hours with it.
Searching the net I found something like the MX-P50M which would perhaps be a choice.
I deployed an Icom IC-703 on one of my activations. The IC-703 has a built in tuner and puts out 10 watts on transmit. I was impressed with the rig. It tuned my end fed antenna with no problems and I only used a 6 amp hour battery. I borrowed the radio from a friend and would not hesitate to buy a used one, unfortunately the radio holds it value and sellers get what they want for them.
5w from an 817 is all the power you need for SOTA /P. Resonant aerials are best for /P to keep the volume and weight down, and the efficiency up.