It’s a difficult decision in many ways Mark. My view is you want a SOTA station that is built out of multiple components so you can pick and choose what to take for each activation. If you have a big heavy radio, you have to carry that on every activation. If you have a lightweight radio you can always add an amp and its battery and decide whether you need the amp or not and save weight.
As you have an 817, an outboard amp would make sense. As most have said, a 50W amp gives the best signal boost vs. weight penalty when you consider you need extra batteries. There are nice second hand Japanese items from Tokyo Hi-Power that come up on eBay now and then. There are some nice Chinese ready made amps too. There are complete kit amplifiers about if you want to make something but probably not suitable for a first project.
There are lots of cheap ways too. PA kits on eBay are under £20 but they need boxing, relays, control circuits and most of all, output filters adding. Do not be tempted to use one of these, or a CB amp unless you have an output filter. We amateurs often complain about interference to our bands from domestic goods that are poorly designed. So we need to ensure we don’t produce interference ourselves!
Finally, don’t forget that 5W of CW has about the same “ooomph” as 85-90W of SSB. CW doesn’t weigh extra or need its own battery. You just need to learn it. Something to consider rolling into your journey to a 2E and thence M0 licence.