Some smartphones (Samsung Galaxy S7 and up) have heart rate monitors, although you might not know it. It's a red LED behind a lens next to the flashlight, which uses the same PPG technology as the Oura ring. You can download an app like Welltory and take measurements during the day, although I don't know how valuable that is.
The Oura ring is a brilliant little device and well marketed, and is the most comfortable to wear overnight, however some health podcasters who aren't sponsored by them have expressed doubts about its accuracy. For what it's worth, I've worn mine for weeks or months at a stretch out of a principled desire to know my biometrics, and found that it didn't give me many useful insights that I didn't already know. The main correlation that I found was the later I go to sleep, the less sleep I get. Common sense, but still striking to see it spelled out.
I know a sleep researcher at Fitbit who swears that a ring will never be as accurate as a wrist device, so I bought a Fitbit charge 4 and wore it with the Oura ring to compare results. Over a week or two, they were both directionally in agreement relative to themselves, but pretty far apart relative to each other. The Fitbit wrist strap is nice and soft, but still more intrusive than the Oura ring, and I stopped wearing it. However I might run the experiment again since it was pretty short. All the spreadsheet logging is kind of tedious. Unfortunately they don't report HRV the same way so I can't compare that metric directly.
Anyway, the point is, don't let one device's opinion on your deep sleep or REM sleep keep you up at night. But the relative results can be useful to discern cause and effect, if you can be diligent about logging your data and subjective outcomes every single day.