Note that Opera GX's CPU limiter is just a fancy way of setting the CPU affinity for its processes. You can manually do the same thing by right-clicking each Opera process on the "Details" tab in the Windows task manager and choosing "set affinity". This manual method will work for regular Opera too.
Also, see https://answers.microsoft.com/en-us/windows/forum/all/how-to-permanently-set-priority-affinity-with/4e83fd39-34a7-49fe-a54a-ee891c38b737 for how to create/modify an Opera shortcut to automatically set affinity for Opera.
There might be something similar for macOS.
For RAM, there some alternatives at https://superuser.com/questions/1263090/is-it-possible-to-limit-the-memory-usage-of-a-particular-process-on-windows that might work for now.