My recommendation would be, have two machines: new hw for all your services, and use the old hw for your NAS. Each could be whatever OS you’re comfortable with using. Most everything on the services machine could be in docker configs, including network mount points to the NAS. You might be able to get away with using the 1080TI in the services box depending on what all you want to do (AI stuff, or newer stream transcoding requirements may require newer hw).
Moving the data from the old NAS to a new one without new disks will be a challenge, yes.
I have a TrueNAS box and used jails for services. I recently set up a debian box separately, and am switching from jails on truenas to docker on debian. Wish I had done this from the start.
Afaik the cookie policy on your site is not GDPR compliant, at least how it is currently worded. If all cookies are “technically necessary” for function of the site, then I think all you need to do is say that. (I think for a wiki it’s acceptable to require clients to allow caching of image data, so your server doesn’t have to pay for more bandwidth).