I've recently learned a lot about this & I'm not seeing the biggest reveal I had.

M/S EQ is good - this is essentially what stuff like Isotopes Imager does. Delaying one side slightly is not as good.

The biggest thing I've learned is multitracking.

You record the same twice - slightly differently each time - & then hard pan one left and hardpan the other right. You can also duplicate a midi track of a digital VSTs, but if you do it's important to make some slight changes to timbre. I've been doing that mostly by EQing, distorting & chorusing each side a little differently to the other. This is important because if you have two identical tracks panned in this way, it's not going to be any different from mono. A digital VST is probably going to perform identically if you put the same midi through it each time - unless there's some randomization aspect.

M/S EQ can't turn a not-very-wide sound super wide. With multitracking I'm usually doubling back and making things less wide in Imager because they're so wide to begin with. You can completely stereoify a mono sounds.