Or you might need to access various character sheets (including monster stats) all at once in various windows
It does this.
providing basically a glorified form-fillable PDF character sheet
This is where I am confused by your position the most. It sounds like you're not even describing DnDBeyond. That's nothing like how it operates. It's much more similar to what existed near the end of 4e's active lifetime than that. Is it possibly you've ONLY explored the "my creations" section of the toolbase?
Having to sign in with two accounts (DNDB and Twitch).
This is not a thing.
I'll tell you how I use DNDBeyond the most often. My DM subscribes to "all content" (called the Legendary Bundle, which sometimes goes on sale but the normal price is 15% off everything). And with that he can share it out to up to 36 other people. So the cost per person, if they combine funds, is negligible (even at the full non-sale price right now it would be under $20 per person for everything if you all went in on a Legendary bundle, though admittedly it's unrealistic to expect you to find 36 people to share it all with). This would get you everything WOTC has ever published for 5e, and a couple of third party products too.
I then added a small tag to my Chrome browser so that any time I type the letters "dnd " followed by whatever it is I'd want to search for in any published D&D book in the address bar of my browser, it will automatically use the DNDbeyond toolset to search out and display that thing.
So if I want four different monsters and three different magic items and a class and subclass and the description of a Xanathar NPC from a particular adventure and a description of a room from an adventure all at once, I can do that, all in different Chrome windows, very quickly. You can open it in different windows as easily as you can open different Chrome browser windows since it literally is that. And of course I can run my character sheet also, and while using Beyond20 I can use that character sheet to directly roll anything in Roll20.
I get that DNDBeyond has some learning curve to really unlock it's potential, but once you get it going it's really an extremely powerful toolset. It's far beyond what you seem to be describing.
You can see some of this in this video starting around the 9 minute mark. Worth checking out, and keep in mind anything you see can just be opened in a separate tab on your browser simultaneous with everything else he does: