Once your current phone reaches end of life, it's worth considering just keeping it around as a dedicated phone while you upgrade your daily driver. It's hard to justify spending 2x the money to buy two phones, but once you end up upgrading down the line, you might as well, y'know?
Google had a 33% off sale recently, I upgraded my 4a 5G to a 7a and am keeping the 4a for games and multimedia (headphone jack baby), play services and other potentially invasive apps in the future if needed. It won't be getting proper security updates anymore, but that doesn't matter because you won't be using it for important or sensitive stuff. You can go further and isolate it on your network at home, too; I connect mine to a different network with no lan access to mitigate some of the risks of having an insecure device connected.
Of course, this isn't perfect. Your provider already knows this was your device, and they know the new device. Certain online services will already know the old device as well, and can correlate between the old device and the new device. but it's still better than the alternative of running everything on the same device.
If you need absolute privacy and can't afford the old phone being correlated to the new one in any way, then you'd have to get rid of the old phone and get a different one (and ideally, never let it be seen by cell towers in the same location as your new phone.) but this should be good enough if you just want to compartmentalize and mitigate the risks of websites or apps being able to "see" or recognize each other, through fingerprinting or any other method they may be able to use.
I mean, you could also possibly manage to sell the old device for a couple hundred dollars, but if you're not that tight on money and are interested in beginning to compartmentalize, that would be a good opportunity to do so without needing to spend too much extra money.