Which Modern NAS?

As always thanks for your thoughts.  So the Terramaster is very interesting because it runs other OS completely and easily, and has great hardware options.  Unfortunately my understanding is that it does not even support ECC at all.  I'm not sure how important that is, but I would prefer to have ECC.  Otherwise it can run any variety of linux and windows11, has two NVMe slots in addition to HDD bays, a GPU, etc.  its pretty nice piece of hardware, especially if I get one of their higher end ones.  

Ugreen I'm nervous about for various reasons.  I have heard hit or miss stories about it, especially when trying to run other OS.  Synology is out for the reasons you mentioned, it's great hardware though, but I'm not sure it can easily run other OS either.  Some limitations there.  There are a couple other notable's, Qnap supposedly is easy to run other OS, so I will be looking into that one also.    I want one that can run other OS as easily as building a PC from scratch, in other words, no limitations or special hacks needed other then perhaps booting from USBstick at the worst, but when they have built in NVMe, then can boot from that even better.

I do want a NAS with raid to store stuff that i rarely access but sometimes do and I want raided to protect against bit rot more than anything.  it all gets backed up 321 style anyway, but bit rot is s real thing.

I prefer to run a few apps on the NAS only because I don't want to run them on another PC.  Plex for example and a few others.  A NAS will be on 24 7 and ideally using less power then my desktop PC's and my desktop PC's I don't want burned with also running those file services.  Either I have to run a NAS plus another dedicated PC to host those services...or I need to put it all on a NAS, just one device..I prefer the latter.  Readynas was handling it for me before no problem, it wasn't great at torrenting, but everything else was fine.  Now I can't due to apt repos down...so...that ends that era.....  Otherwise if I just needed simple file service, I would probably just keep using this 524x for the foreseeable future.

I also don't actually want to have a whole PC running all the time, I am mostly Macs here, but every once in a while it's handy to have a windows pc available.  A NAS with win11 VM running can accommodate that occasional need.  The readynas could not ever do that, but these newer NAS, some of them can handle that, on top of running Truenas or whatever...my various file services...and a virtualized win11 for the odd windows need that might come up.  Then I don't even need to have an entire PC running there doing nothing most of the time.