Lots of really good questions here, which I’ll do my best to answer without going too long!
I think both of the ideas you’ve laid out here would be sound, especially given the fact that you’ve got three years to work with. The nice thing about having that much time is that you could easily try one approach and determine after a few months whether it’s working or not, and shift gears if necessary. What I would encourage you to do is create a few benchmark sessions that you can use to gauge progress (the selection events, or variations therein, would be a good start). If you find that your performance on those events is improving with your current plan, then carry on! If not, make some small tweaks and then reassess. Three years is a long enough time where it becomes hard to simply say “do this” because you’ll inevitably become stronger/fitter/better along the way and that development may require a different approach. Hopefully that makes sense…
Two things I would emphasize throughout your training: loaded movement (ie rucking) and weighted carries/grip work. Both of those elements are crucial in selection events.
As for the strength work, I’ve had a lot of success with guys focusing on compound movements with a secondary emphasis on unilateral strength. Unless your selection course has an absolute strength criteria, we want to be careful about becoming “too strong” at the expense of aerobic development. All that really comes down to is managing your training variables…something that individual coaching might be good for.
Hopefully I answered a few of your questions here. Feel free to reach out to me (firstname.lastname@example.org) individually if you want to dive deeper into anything. Happy to help!