The questions you’re asking simply cannot be answered specifically because it really depends on a variety of factors.
The requirements for TT-RSS are a dedicated or virtual dedicated server, and for good reason as TT-RSS can be fairly intensive. There’s a lot of information going into the database and an ill-equipped machine (like a Pi) will choke eventually. PostreSQL will perform substantially better than MySQL/MariaDB.
How frequently you purge articles will also factor in because some people (for whatever reason) never remove old articles or remove them after a year. Obviously this keeps the database larger.
I would be incredibly surprised if your Raspberry Pi were able to handle this setup after a few months. If you’re using a cheap SD card fpr stprage you should also expect it to fail as well as the write cycles on that storage medium are typically less than conventional drives.
I would like to point out that in the last few years prices for VDSs/VPSs have dropped dramatically. You can get a reasonably spec’d virtual machine for just a few dollars a month. In addition to having a more powerful CPU, you will also get a good amount of RAM, a fast SSD that’s probably configured in a RAID, support personnel to keep the hardware running, greater bandwidth, greater traffic.