1. I've never personally done it, but apparently PostgreSQL is possible...
Running postgres on synology box - Synology Forum
2. Yes, the Synology DSM supports iSCSI so you can share it that way if you like.
3. No, my NAS is on 1GbE and 10GbE only.
I've only read about the performance issues of Arm based Synology NASes and have never seen it first hand... With XPE, you can construct a system as powerful as you need. On their forum, there are users who are running powerful Xeons. I chose the Avoton C2550 for its low TDP and it's ability to transcode in Plex effectively.