DO NOT use the SD Card for any sort of heavy read/write data like a database. SD Cards are not meant for such use, they corrupt easily and flashing / rewriting the data does not work. If you're going to use a Pi for a relay then get a USB drive with a few terabytes of space and write to that. It will be speed bound since the Raspberry Pi shares the Ethernet Port bandwidth with the USB ports but a small relay should probably be fine if you manage the amount of users. 

I don't know what model of Rasp Pi you have, but you should attempt to get the operating system off the SD Card as well. 

I can't comment on connectivity issues, I don't know anything about Nostr's stack to give advice beyond check that your ports are forwarded correctly at the router level and if you're even able to connect to your house, many ISP block inbound connections on all ports.