No prob. 
I’m not 100% positive that all this work is going to end up with a 100% functional/operational dual monitor setting (because of how recent the Intel UHD 750 GPU is), but it’s going to be satisfying if it does. The kernel upgrade already made the system boot faster and automatically load the i915 driver, so the effort is starting to pay back. 
Yup, I looked some articles about the OptiPlex series and 5.14+ are the ones that work best with OptiPlex 5090 (besides 5.14 being specifically recommended by Ubuntu’s metapackage for Dell hardware/machines). I thought that 5.14 was available as kernel-lt but your reply confirmed that it’s part of ml. Ok. Hope a newer kernel solves this. 
I just found this discussion about i915’s parameter force_probe:4c8a to enable Intel UHD 750 GPU support and it seems that, indeed, adding such parameter to GRUB really is the way to go if upgrading the kernel with modeset enabled doesn’t solve it per se.