We had a very similar path. I was able to get a free upgrade to a Pixel XL with my Nexus 6P. Then I got a OnePlus 6T. I got the Supcase for the OP6T, but just wasn't a fan compared to the Nexus 6P version. Finally got the S21 Ultra this month and bought a bunch of cases, but settled on the Ringke Fusion-X.

Now that I'm looking at the Supcase listing on Amazon you are right. It doesn't have a screen protector. The separate front frame made me think it did. I must've got it confused with the Poetic case that does have a screen protector built in.