Q.Jade plant is drooping but has ideal conditions
My jade plant gets lots of indirect sunlight and has thrived where it is over the last 10ish years, only over this winter has my plant decided to droop. My house can get warm through central heating and my wood burner downstairs but generally, good temperature and humidity levels tend to be around 60/70% through out this time of year. I have never over-watered it and to be sure, recently I checked for root rot and found nothing. What is even stranger is that there is still a very healthy amount of new growth that’s appeared over the last year and still some more recently.
ALSO when I checked the roots for root rot I discovered that the roots were very brittle and just fell off. The plant must have lost between 1/2 and 2/3 of its root base (2 weeks ago) the plant has been Droopy for months.
But over this winter I barely watered it at all, say twice over five months, and I think this is a big factor. It now looks like your stereo typical drooping Jade plant, I think there’s a chance that it is very dehydrated due to it being a water based plant. I do have younger jade plant that is thriving, only a few years old (in background). I have been cutting off any dying branches and leaves with sterile scissors.
>Watered 2-3 times between Sep-2018 and Feb-2019 and only smallish amount
>Droopy for months
>2 weeks ago I checked for root rot and re-potted it. Root system almost completely disintegrated Since then it has gotten a bit worse. Is there any way I can try and repair it, and would there be a timeline and time of year I should start? Hopefully it can recover.

Have you re-potted with fresh potting soil in those ten years?
Does the pot have drainage holes? I don't see a saucer so maybe there is an insert pot inside the outer pot. If so the inner pot is too small to sustain growth any longer, and maybe the nutrients are depleted from being in the soil so long. You don't mention a fertilization program.
If the outer pot is the only pot and it has no drainage, than that needs to be corrected immediately. It would be my first priority, to repot with new soil, and use a larger pot with multiple drainage holes, and a saucer so that you can monitor the saucer for excess water and know that it is too much. Use the moisture meter also, to help take the guesswork out of when to water.
Use a slow release complete organic fertilizer in spring and fall.
Repot and shape back the long leggy branches so that new growth sprouts closer in and achieves some density of the foliar crown.
Here's a healthy 8 year old jade plant:
https://garden.org/thread/view/28312/My-Jade-plant/
https://www.gardeningknowhow.com/houseplants/jade-plant/jade-plant-care.htm