Update: over the past couple of days I have tried to classic crapshoot approach of doing the following: filling up with 98, cleaning the throttle body, spraying in a can of upper engine cleaner into the intake, plus an oil and filter change.
One thing I noticed when doing the oil change was that only about five and half litres came out, a litre less than the six and a half that I had previously filled it up with. I suspect this may have been recent, as I don't recall noticing it that low the last time I checked the oil level. I have kept an eye out for leaks and I have never seen one.
Upon taking for a drive, things seemed more or less fine and the engine was responsive and seemed stable enough. I got to speed on the freeway and felt some roughness when getting up to 90-100 (probably around 4k RPM or so). It seemed to stabilise quickly and I continued on normally.
Back on the streets, I tested out the AC again and it definitely felt a lot better. Still felt a vibration or two here and there, but not the same level of frequency or the noticeably dip in RPM when idling. One thing I did notice was that the heat from the engine through the dashboard vents might have felt hotter than normal when the AC was off, however it's not something I've really noticed that much before so don't have that much of a reference point. Oil temperature gauge is in normal range.
Overall, this combination of approaches probably did help out a bit with things and are all routine maintenance things I would be doing anyway, but I can't say that it's really got me any closer to figuring out exactly what is causing the issue. Considering the potential increase in engine temperature and burning of oil I'm suspecting it may be coolant related, but I'm not sure as this isn't reflected in the oil temperature gauge. PCV issue maybe?
