Shopify General Check up and Optimization

I represent a company who's brand will launch on the 6th of February, and to coincide this we would like our website to run as smooth as possible. During development we have noticed some issues:

-The mobile layout of product sections inconsistently breaks on some devices, but when run through
simulation works fine, so we don't know what the issue is.
-Customers have issues setting up accounts, again an inconsistent issue so difficult for us to test.

We would require someone with knowledge of Shopify to go through the code of the website to find out what the issue is and fix it, as well as do a general check for any other potential issues.

The website in question is here: https://gooditsglutenfree.com/

Thank you
Michael Elliott