My first real NAS was a Synology 920+, which I'm still using after several years. I knew nothing about NAS', and it's been amazing!
I'm self-hosting numerous applications via docker; have replaced quite a few paid services (google drive, Evernote, etc.); and it's already come in handy to retrieve deleted documents from my wife's computer (several times!), which we back up to the NAS.
My only real suggestion is to go with as many bays as possible. Mine is a 4-bay, and I've already filled AND upgraded the capacity of several drives. I wish I had gone with 6 or 8 bays, but I really had no idea that my NAS would be this versatile.
I do also have an 4-bay external HDD enclosure, but it only serves to make local backups of my NAS. I see my NAS as the main hub, and place for ALL of my data, so external drives are either feeding data to my NAS or storing backups.
I connect HDDs and even an action camera to the USB port when I need to make backups of my NAS, or to copy data into my NAS. I haven't needed to reformat anything just to work with the NAS.
my docker use is pretty basic: rss aggregator, podcast storage, bookmark storage, etc.
Drives don't make any noise, other than the typical "HDD read/write" noise that you'd get from a spinning HDD. The fan can be adjusted to different levels, and rarely do I ever notice them.
I expect any NAS to last as long as a typical desktop computer: at least 10 years. Drives can be swapped out and replaced when needed, but this is far easier to do the more bays you have, since you'll have redundancy in at least one other drive. Theoretically, I can lose an entire drive and not lose any data. In fact, even upgrading a drive is very seamless, and the data gets rebuilt into the new drive. Pretty cool stuff.
I don't use syncthing. On my computers, I'll sync/backup local files to the NAS using Synology drive. On my phone, I have Synology Photos backing up photos and videos, and Synology Drive backing up my "documents" folder. I do also use SMBSync2 (Android) to copy music files from my NAS to my phone, so I can listen offline.