Retrospectives look backwards, and most teams aren't wired that way. And unlike sprint retrospectives, architecture retrospectives turn up issues that are unrealistic/impossible to adjust in real time.
Instead, try lightweight architectural decision records, during the selection process, at the start, when things are easiest to change, and can be the most flexible.
When teams try ADRs in practice, teamwork goes up, choices get better, and implementations get more realistic. And if someone later on wants to do a retrospective, then the key predictive information is right there in the ADR.
https://github.com/joelparkerhenderson/architecture-decision...