Yes, we have plans to support PostgreSQL as a storage backend. At the moment, queue tuning parameters are not exposed as configuration parameters, they're based off a shared queue and we expose prometheus metrics over the shared queue. What use-cases do you have for tuning the queue?
So essentially Convoy's queue mechanism works as a delayed job queue (basically how retries in webhooks work e.g. Stripe will send the event to you every hour for the next 72 hours). When the endpoint is unaccessible it backs up and schedules the retries at a future time on the queue.
The implementation under the hood is a combination of redis zset + redis streams. :) When it maxs out retries the events are still stored in the DB and you can batch retry them once the endpoints become accessible.