my question is if it is possible to assign a new address to a customer in beforeSubmit or I need to program the after submit instead and reload the submitted record

I tool a peak at a script I wrote to auto-add addresses to customers, for whatever it's worth... i wrote it in Dynamic = true and in afterSubmit

i ended up doing that

sublist data access certainly does not require dynamic mode as a general rule. I almost never use dynamic mode in server-side scripts.

but it should be in after submit right? or you say it should work in before submit. Regardless the dynamic mode set to true

I don't remember for sure if it has to be after submit in this case, but what makes the situation a bit unique is it's not sublist data you're trying to set but rather subrecord data.... so it may indeed require aftersubmit since the subrecord needs to be associated to a yet-to-be-saved new parent record.