If you can replace an M4 Pro workload with an i5-9400, why on earth would you have ever purchased the M4 Pro? Even an M1 significantly outclasses an i5-9400. And I have family members that have been using different 24" Dell P-series displays in my house with no complaints, but those monitors don't even compete in the same zone as a Studio Display.
It sounds like you may have severely overpurchased when shopping for your first setup. Reminds me of a friend of mine that bought an RTX 4090 literally just for Diablo 4 (and then bought a replacement 4090 when his first one fried!).