https://e.foundation/e-os/ can just buy these. Essentially LineageOS with some tweaks

Galaxy S9 is something like $100 on eBay and well supported

Maybe https://github.com/citusdata/citus#what-is-citus ?

Might be a fun project to work on. I wonder if we could convince some grant funder to give us money?

Would this approach help with the database replication? https://hevodata.com/learn/postgresql-sync-replication/#Method2

Old laptop probably best, ARM is getting better though. I’d be tempted by https://forum.banana-pi.org/t/banana-pi-bpi-cm5-computer-module-with-amlogic-a311d2-4g-lpddr4-ram-and-16g-emmc-onboard/15578/3 but expect some software hassle