Where can I learn how to manage a custom wordpress site?

I have a link to a free course which should be extremely useful. I often refer my clients to enroll
https://www.udemy.com/wordpress-basics-how-to-manage-your-website/

It really depends, what do you want to manage and who is gonna build the custom theme? Depending on your requirements you can prepare the theme to be more or less flexible and therefore you can get more or less control of it. I'm happy to go into details if you specify what exactly want to achieve.

If you are looking to improve the performance of the website, you shall use GTmetrix, PageSpeed Insights or Pingdom Tools. This can help you to improve the performance of each page in your site built on themes like Avada, Divi etc.

If you are looking for other customizations, then I need to evaluate those. You can setup a call for discussion so I can guide you professionally.

First question is should you DIY it or hire an expert.

This revolves around your site income. If your site is producing any income at all, question becomes what's your best daily activity...

Developing content... Marketing... Other profit generating activities... or mucking about with WordPress...

Tip: First find someone who understands WordPress.

For example...

Avada - Never us this theme because it's massively bloated + will cause many problems when your project succeeds + traffic arrives.

Divi - The worst of the worst. This theme is short code based, so to change themes, you must do a full site retool. Also, Divi uses a poorly conceived + implemented caching system for it's internally generated CSS files. As traffic increases, this process breaks down + sites become scrambled.

Tip: First step of WordPress site management is to select a development stack (theme + builder) which works correctly, independent of traffic load.

Stick with GeneratePress (theme) + Beaver (page builder) + you'll avoid many WordPress problems.

The other massive problem managing WordPress sites is most hosting companies... sigh... competency seems to revolve around a game of "how low can you go".

Primary reason WordPress sites get hacked is hosting companies incompetent approach to managing OS level package updates + service security... meaning every connection into machine is secure. Period. No exception.

Might be good for you to scan Clarity for people who work with WordPress + hosting... all day... every day... then book calls with them for direction.

And... if your site is generating income, mucking about with WordPress is far better handled by hiring someone, rather than you learning all there is to know about hosting management + WordPress management.