On my Corretto first crack is around 210C. Second crack around 230C.
In most posts I have seen people are getting first crack at 205 and second at 220+.
I dont think you are getting a high enough temp.
A lid will help but it comes with its own issues, roasts can be smoky if you arent careful.
Try lowering the height of you heat gun or lowering the batch load.
Are you able to post any pictures of your setup?