Lots to unpack.

How much of your Fuji dissatisfaction is a RAW converter/processing issue? How much of your forest understory challenges could be resolved with faster lenses?

If starting fresh a mirrorless kit with weatherproof body and lenses in whatever sensor format you prefer would give the best combination of performance:weight. Can you adapt your Nikon lenses to their new mirrorless body? You don't say whether they're APS-C or 135 format.

I've retired my DSLRs but continue using some of the lenses in a mirrorless environment. However, I stick to mirrorless lenses backpacking because they're much smaller. I necessarily pack the camera gear last, so what I take varies, depending on how long and challenging the hike is. Never been lucky enough to have a porter!

Also, I assume batteries and charging opportunities are an issue. I pack batteries based on a daily shot budget. Storage cards are so big now they aren't a limiting factor.

Cheers,