Learning to manage existing WordPress/Divi site

I'll try to keep this concise, but find it hard to organize my thoughts on the subject

My problems are as follows:
1. I have a "complete" site designed by a friend, but it's not converting leads with Google Ads. I'm getting plenty of impressions and a decent number of clicks but nobody is filling out the lead forms. I suspect the web page design could be improved

1. I'm eager to try different landing page options but have to pay and wait for each revision. My web dev isn't a full-time pro and seems to have odd problems with aligning things properly. The landing pages I design are all simplistic and I'm confident I could design and build them myself, eventually

2. I have access to the wp admin panel, google analytics, etc. Yoast is telling me my SEO is poor, analytics is telling me my page speed is poor. I don't know how to resolve these issues and I suspect my web dev doesn't exactly know either, nor does the have the time to dedicate to this. Things like getting a google code snippet added seem to be beyond his skill level

3. I've taught myself excel, illustrator and lots of other programs but find the wp admin panel I see overwhelming and unintuitive.

Would it be easier to learn to build a site from scratch or to learn how to modify an existing site? I'm very confident I can figure out a page builder whether it's divi or elementor or whatever but intimidated by the backend stuff like connecting my domain to the site, managing SEO and making any code changes