Note that t3.large is burstable and so performance isn't predictable.
Independent of that, a machine with lots of RAM won't help because node (which Central uses) won't use more than 2 GB of RAM unless you specifically allocate more RAM.
See https://docs.getodk.org/central-troubleshooting/#reading-container-logs for how to read logs and check status.
See https://docs.getodk.org/central-install-digital-ocean/#increasing-memory-allocation for allocating more memory to the service container.