Thank you for the help here, I was able to get things working well, and wanted to post a follow up response for anyone wondering about this, and maybe a separate post later. All directions found here: https://troypoint.com/troypoint-toolbox/

Using the Downloader app, I installed Wolf Launcher 0.1.9, then Launcher Manager 1.1.9 for Fire TV (which also replaces and/or redirects to the default settings menu as needed), and followed the tutorial steps to set Wolf Launcher as the default for the home button. (Of the four launchers in the toolbox, Wolf Launcher had the fastest startup, and once I understood the settings it was easy to hide widgets, sections, labels and unused apps from the view for a simple clean grid of all my apps on a dark background that doesn't scroll across the screen or anything unnecessary.) I wasn't able to install Automate (that I could tell), but instead installed Launch on Boot 1.13 (12), and used it to set Wolf Launcher to start on boot or wakeup.

It seemed tempermental at first because I was changing settings around, so I guess triggers got confused for a bit. Once I got it set correctly, now it still boots to the original Home screen first (with everything black, blank and/or missing except for the Home and Settings icons, since everything Amazon is blocked via the pihole), but once Home is done loading nothing, it quickly switches to Wolf Launcher. (I opted to not disable or remove the built-in launcher and apps to avoid accidental bricking since this already works, and I could just hide apps that cluttered the view now.)

Using the Home button isn't perfect, but it's definitely good enough. From within an app it often defaults to the original Home screen, but pressing it again once or twice usually switches back to Wolf Launcher, and simply exiting an app normally or via the back button instead will go to Wolf Launcher, so it's not a big issue. If for some reason Wolf Launcher doesn't start, the Settings menu is still accessible so it can be rebooted from there to reset it, or Wolf Launcher can be started from the Settings Applications menu.

Thank you again for the direction, it's finally setup the way I wanted!

edit: and to whoever is reading this, be sure to go into settings beforehand and disable automatic app offloading, and make sure all your apps really are there and don't need to be reinstalled again before you shutdown all communication to Amazon and the App Store. Even if an offloaded app is visible on Home, it won't be visible or installable from the other launchers. They only see installed apps.

edit2: also be ready to block access to 8.8.8.8 (Google DNS) in your router settings, because FireTVs have that as an automatic entry that you cannot delete. You can set your own DNS to a pihole for domain blocking, but separately you'll have to block 8.8.8.8 or it will always have a backup path through. The only way I've found to know it's blocked for certain is to use the Downloader app browser, and try going to 1.1.1.1 first to see Cloudflare's website work, and then go to 8.8.8.8, and it should be inaccessible if you have it blocked correctly.

Thank you for the detailed info and the tips, it’s much appreciated! With that to lead me in the right direction I was also able to find a possible solution for the home button without a workaround using an updated app, though it seems ymmv depending on whether or not Amazon can automatically reset it somehow. I have pretty much everything blocked so the home screen is usually empty except for my top row apps, so the button fix may work for me, but those workarounds may still be needed. Thanks again!