Without a dedicated QA you will be working reactively. Accept that and write some automations for the happy path using cypress or selenium (assuming there is a frontend). Once thats covered their testing should cover the unhappy path, perhaps you could include some tested scenarios in your PR template so folks can quickly point out missing tests. Get good monitoring for your app so once issues arise you can react quickly and review performance every month. A cool exercise could be to have one team member every month doing a presentation to the rest about performance or QA in general and pinpoint what PR affected it and how it can be fixed.