Need advice for Wix SEO

I'm a Wix hater mostly, and often refuse any work on Wix or offer a change to WordPress to my clients. But the day came when I can't do that because is pretty expensive and just wrong with this client. Making a change to WordPress would not help... So, I need to learn about Wix this time and overcome my hate to the platform.

So please advice me, if you have experience with Wix sites, how should I approach SEO?

The site so far has made all the basic optimization Wix allows doing (alt text, descriptions, and few very basic things) Next for me is optimizing content, because it is very poorly developed.

Can I change the sitemap.xml?
Can I change robots.txt?
Change URL structures?
(all if needed)

What are the most common struggles with Wix nowadays?
Is there something I should know before getting my hands to work?

Anything you can tell me will help. I'm ignorant in Wix for now.
Thanks.