I'm ngl this is surprising to me, as GOS has always just worked out of the box the way I wanted it to for me.

But:

I’m facing the nearly insurmountable task of convincing my friends, family, and colleagues to download and use signal when they are all using encrypted iMessage.

Anyone who uses Android will experience this. I've never owned an iOS device in my life and I've always used SMS and Signal to talk to people. Have occasionally downloaded WhatsApp when a group of people insists on using it and I need to communicate with those people, but usually WhatsApp is uninstalled when I don't need it. I think most Android users just use WhatsApp though.

Most of my banking apps just simply do not work.

Even with sandboxed Google Play? Again, surprising to me tbh. All the banking apps I've used in the past have worked fine on GOS without any Google Play services, though I don't have any mobile banking apps installed atm. I second the other commenter who suggested switching banks if that's possible for you.

There’s also a few features that I’m assuming are iPhone exclusive that it really sucks to have without. Double tapping the bottom of the screen to shift everything down so you can reach the top of the screen with your finger when using one hand. Holding down on the space bar to move the text cursor between characters. Maybe these exist on gos though?

I'm sure you're not the only person who's switched from iOS to an Android-based system and misses these features. A custom launcher might have the former feature, and there must be an Android keyboard that offers the latter. Maybe ask around on more mainstream Android forums, as they'll probably have the most people switching from iOS to Android.

No clue about Yubikey, sorry. Never used it.

If you want to use an iPhone, you can. You don't have to use GOS. I understand if you've invested heavily in the Apple ecosystem, it's just inconvenient to stop using it all of a sudden. Ironically I sort of experienced something similar in reverse when I tried to daily drive Windows for a brief time because of gaming, and I found it so frustrating to not have access to a lot of the programs I used on Linux, and how things worked so differently (and in ways I thought were much worse) on Windows. Not quite the same since there's definitely no such thing as a "Linux ecosystem" in the same sense as an "Apple ecosystem" (good! I don't want to log into my online Linux account to boot my kernel...), but big changes to your tech workflow will be frustrating as you build up a new system that works the way you like from the ground up. I don't think using GOS as a daily driver is a necessity for everyone. I would like to promote people using degoogled, FOSS, privacy-respecting OSes both mobile and desktop, but ultimately, you are an autonomous human being and can use iPhones if you prefer to do so and are fully aware of the privacy issues.