What i need to do to backup and secure my mobile data before sending it to repair
Apple gives instructions on what to do before a repair, bit it should really just be this article.

https://support.apple.com/en-us/HT201351

I prefer that checklist since it talks about Apple Watch and health data backup. You can not get that data back, so by following that before you sanitize your device, it’s protected. Ship the device insured since you can’t have Activation Lock enabled and follow the process.