I personally believe that Upper/Lower splits are some of the perfect balance between frequency and volume. Keep the rep ranges and exercises rotating so that you dont stagnate.
Also, you may look at what exercises and reps you are using in your full body. If I rotate exercise planes and the rep range so I am not doing the same thing each workout, I stay fresh regardless of If I am on full body, multiple daily sessions, or a split.
You may try 4 weeks on U/L split or Push/Pull/Legs, and then return to full body afterwards.
A good option is to emphasize other parts(more volume), while doing some work for the whole body. This way you'll get the benefits of splits, but still maintain or raise your work capacity. .. You may consider them fullbody or split sessions. Either way you'll get some work for every bodypart every session, even if it's only some couple of light sets not taken to failure for the non-emphesized bodyparts. ..
I think that as far as hypertrophy goes, the more work you do for one muscle group in one session, the more sacroplasmic the growth is going to be since your fast motor units fatigue earlier in the workout as opposed to shorter more frequent sessions producing more contractile hypertrophy. So neither way is wrong. But seeking more contractile/myofibrillial hypertrophy is propably more achievable for natural trainees.
It is pretty much standard PL split and it works very well.
As for you feeling much better after switching to split training, have you considered the possiblity that you just needed a week off or a deload? And that dropping frequency, by using a split routine, has essentially provided just that? Just an observation