I use MAMP locally for all of my Backlight development. I have Apache selected as my web server, and PHP 7.2 running at the moment. I don’t think I’ve had to do much fussing to get it to just work.
So, what if you stop using HTTPS for your local development? I run MAMP Pro as http, and with a blank port (so default, I guess).
Nice to hear you’re doing well with MAMP @Matthew. You’re just making me jealous 
If you look at the whole of this discussion you’ll see I tried the http approach and still had problems.