@Kaldo It's a support thing. Maybe even consider Syno's "own" HAT3300 HDDs. 4TB has an ok price, at least where I live.

You can run Docker on a Syno NAS via DSMs Container Manager package perfectly fine, as I do.

Keep in mind, the DS233 has an ARM CPU with less "grunt" vs. the 224+'s Intel CPU and can not be expanded beyond the soldered-on 2 GB RAM.
But, while it has been a backup device/test mule for years now, I used to run Docker containers on the DS216+II even with a meager 1 GB of RAM.

@Kaldo
Syno DS224+. Good bang vs. buck. Decent speed, decent features (iGPU, Docker, VMs).
My only gripe(s): only 2 USB ports, no built-in 2.5 GbE (possible via USB and 3rd party driver) and Synology's limited list of "approved" HDDs.

1. Yes. If your HDD is formatted as NTFS or exFAT, no reformat _should_ be necessary.
2. What do you mean by docker drives?
3. Mainly the drives.
4. My DS216+II runs 24/7 for 7 years now without hiccup (but, obvisously, YMMV). DSM supports drive migration.