What’s the best way to go from a design to a customized WordPress site?

I don’t have enough rep yet to comment so I can only post as an answer. Based on your particular situation, I think any well supported free theme from the repository that is close to what you have and that takes minor tweaking OR a one-time $39-$59 theme would save you a ton of time and frustration trying to manipulate a default WP theme (block editor is not for everyone either if that is where you are having issues.)

Yes, a developer could build it but, since you are doing this for a friend and trying to keep the cost down, I would save some frustration and look at another option or go HTML/CSS as you suggested.