Anyone have experience buying/using a Sim-free phone? (and maybe more specifically, with a UQ mobile Sim card?)

TL;DR I’m thinking of buying a new phone, but I’m pretty happy with my UQ mobile service. Anyone have experience/troubles getting it to work?

Now for the details cause I’m sure someone would ask. Simply answer if you like without reading.

4 years ago, I got an Xperia XZ3 through Softbank. Love the phone, hare Softbank. My now ex-wife and I got the deal cause they had a special sale for moving from other carriers, you know how it goes.

Fast forward two years. The 割引 ends and we end up paying something rediculous like 9000 per phone. The two year deal ended. We could buy new phones and a new deal blah blah. But I specifically bought the XZ3 because it was a high quality phone that I wanted to keep for a long time.

So, I decided to unlock it (yes, did the whole process of unlocking the phone with Softbank, pain in the butt with a fee, but did it). Ended up going with UQ because they too had a deal when moving carriers with a major discount on a phone. The price of the plan never changes, no limited discount to worry about. So I said sure, go with that. With the discount, I got a free OPPO phone with some data and a phone number. They told me I could use it in my Xperia, but because it is a softbank phone, it can only connect to towers with a softbank signal or something to the such. My UQ Sim card runs on the KDDI towers. There are plenty of towers that share both, but they said I wouldn’t be able to use the KDDI exclusive towers.

So, for the last two years, I have been using my OPPO phone with the UQ Sim card, and tethering my Xperia XZ3 when I go out. Kind of a pain.

Anyway, my Xperia XZ3 is still running great, but I’m just thinking down the line, if I get a new phone, I want a good one but want to stay on UQ and not have any hassle changing carriers, nor the fees and such of changing, etc.
Long explanation, but there it is.