As stated not enough info.
Transformers etc.. work.
Heres a cheap quick fix that should work if you have no problems other than line losses, no bad splices etc..
The gennie running from a water power source should provide a constant power 24hrs I assume. Provided of course it not just when a dam is open upstream and subject to change at times. So if this is a constant,
Put 2 batteries at the power source and charge those, this is to provide amps to the inverter when more is needed for a short time than the gennie produces itself. Use as many as needed. Leave any dump loads or regulators there to prevent those from over charging. Now connect a cheap inverter to make 120Vac to power the line to the cabin.
Move the bigger better inverter and all other batteries to the cabin. At the cabin plug in a Smart Charger to charge those batteries from your AC line. Run the best inverter on these batteries to power the cabin.
Now, since the batteries at the cabin are Smart Charged you do not need a dumpload or anything for those. The batteries at the cabin will supply the power for heavy inverter loads when needed and also any DC loads you may want to add like lighting. The cabin inverter will be suppling 120Vac to AC loads. No more worry about 90Vac to computers etc..
Using a smart charger you will be charging 24hrs a day.
Now say you kick on a 1200watt microwave for 60 minutes at the cabin. That's drawing about 100amps 12VDC (plus) from your battery bank at the cabin. Everything works fine, plenty of power. The line to the cabin though is not trying to deliver that power, it is only running a charger. So if charging at 25 amps 12Vdc the line from the gennie is delivering maybe 2.5 amp 120V AC, it would take 4 hours (plus) to replace the 100amps used, but you got 24hrs to catch up anyway. Course if your only getting 90Vac then you would be drawing more amps at that rate.
Anyway for a quick fix cheap you get the idea. If your daily use at the cabin is say 5kw hrs you need 416.666amps of 12V battery power to meet that need. BUT you have 24hrs to fully charge the batteries and that's an average of 17.36 amps per hour.
You will have loses with the batteies and inverter etc.. I did not count, so let's say 20amps. To do this this way, you shouldn't need much for batteries at the gennie itself. Just enough to power that inverter durring the highest loads the smart charger draws, and the genie cacthes them back up durring lower loads. Maybe the gennie has more than enough power to cover the heavest loads and the battery is just a buffer?? I don't know how to figure that, too many variables unknown.
Now I just bought a Shumacher 2/12/25 amp Smart charger at Wal-mart for a little over $60. My Vector 2/10/20/40amp Smart charger cost me around $80-$100 over a year ago. Both work from a cheap inverter just fine. The Victor I ran at 20 amps from a 400watt inverter fine a whole lot, could not run 40amps though off the small inverter.
I have a old cheap 300watt inverter, The Shumacher runs fine off that at 2amps, but the inverter cuts out if I try 12amps. They both run at their max power fine if I run off my Aims 5KW inverter
Both have a battery desulphate mode which is good to have also.
Maybe a cheap 12V 1000watt inverter for under $100. Both would be usable for other things and nice to have around when you fix this right. So alittle work moving things around, Smart charger and inverter purchase maybe you could get by with $150-$200.
Since you have a 24V inverter now, it makes things a little more complecated as far as moving it to the cabing and charging there. Can still be done, or buy a 12V large one for the cabing and leave the 24V at the gennie.
Another thing uncertain is what does this part actually mean??
" I have the opportunity to buy into a beautiful off-grid cabin (a life long dream) on a communal trust property "
Does that mean other people are involved and you don't own it all by yourself, which might also mean you can't do just anything you want to it? If so, or something like that, you could leave everything as is, set up batteries and inverter in a pickup truck, charge and use those as I mentioned above from the existing line.
I do something sort of like that with a remote rock house I rent at times like now.
I charge at home or off the truck while driving, then use them out of the truck at that house now for now. Last time I had a battery bank there at the house and charged those with a smart charger and the 400watt inverter from the ones in the truck. This is why I know the chargers work off inverters
Sometimes when other people are involved we have to think of other ways. Since I don't own the house I am limited, no puting up towers etc..
Though I am working on a tower system for my truck itself, can be lowered to be drivin, but will mostly just be parked there all the time.
My kid wants the grid turned on there, but I am trying to get off grid at home, why turn on another??