First, make sure you're not using any of the limiters in the GX Control sidebar panel.
Next, make sure the battery saver is off at the URL opera://settings/batterySaver.
Then, goto the URL opera://discards and turn off auto-discardable for each tab you want. You'll have to repeat this with each Opera session.
If you want something that does opera://discards for you automatically, there are a few extensions in this list that will disable discarding. Pick one that says it disables discarding and try it out to see if it works. If not, try a different one that says it disables discarding.
If you still find some tabs snoozing even with one of the extensions that works in general, goto the URL opera://flags/#calculate-native-win-occlusion, disable the flag, and restart Opera.