Disclaimer: most instructors will want you to dive the stock unmodified unit for at least 50 hours(?) before considering changes, and if you show up or struggle on modified equipment they may rightly suggest you revert before continuing the course.
Any backplate/harness/wing combo will work, within reason. You can cut & re-melt slits into the nylon material of the wing center for passing hoses through. SPGs can be run over the shoulders, or switched to transmitters on shorter hoses.
Shoulder D rings can be freed up further by passing MAV hoses through some DIY shock cord loops instead.
I keep the stock thick steel backplate for coldwater diving in a drysuit or thick (7mm++) wetsuit.
Otherwise for travel & tropics in thin wetsuit (up to 5mm), a standard aluminium or composite backplate instead. The stock JJ is designed for drysuit diving. For less-buoyant people, it is too heavy (dense) for thin wetsuit diving. Especially when adding on stage cylinders (except lean trimix ones)
The JJ harness is designed for quick tightening/loosening via the lower D rings, keep using that for ease of use if in a drysuit.
Also for cold water/drysuit diving, consider finding some ~1" wide x 1 foot long diameter lead rods, coating & sealing with heat shrink, and inserting into the tubes below the top handle (unscrew the large plastic nuts)
I haven't needed any head cover, since I steer clear of ultra silty/sandy water and do not habitually bang into cave or wreck ceilings. But I do recommend the screw-on sensor protector, for when you take things apart to air-dry or store. You can also screw a 9/16" threaded plastic cap on the solenoid input fitting to keep out dust etc, provided it is dry inside and free from internal condensation.
The
3.7v rechargeable 14500 batteries can power the Shearwater computers and HUDs (including one of the onboard JJ batteries). Reportedly some can be found at up to 1100 mAh. I recommend to double-check your vibration alerts to ensure they are working, whenever you change batteries or prep the rig. As not all battery types are supported. It probably belongs in the pre-dive checklist now for Petrel 3 units. I still Carry a spare 1.5v energizer lithium just in case. Not sure about the 9V solenoid batteries.
I recommend using a BOV and gag strap on the loop, if you anticipate any challenging or 'self-reliant' diving. I put a Divesoft one on mine. Sanity breaths become trivial to do. With a BOV it is much more likely to make preventative decision to go onto bailout easily and reversibly. The non-BOV drills are all necessary and easy in training, but good luck doing a timely DSV/regulator exchange during a real CO2 hit, panic, or struggle, after already delaying the decision for too long because of reluctance, denial, uncertainty, distraction, or task loading.
Ordinary silicone lubricants are not a fire risk for low-pressure o-rings, e.g. canister, loop, MAVs, etc. Just don't get it around high pressure oxygen (cylinder valves, regulator first stages), or silicone flapper valves. Wash hands & tools thoroughly before handling those.
I bought a Rubbermaid Brute garbage can for submerging the whole unit in fresh water. I forgot whether it is the 44 or 55 gallon model. During rainy season I keep it full and fresh for free via a rainwater diverter from a house gutter (periodic drain & cleanout).
You can build a free-standing rack or wall rack to hang the whole body/lungs/wing/harness/regs assembly upside down to drain & dry after soaking/rinsing/steramine steps. This means you can drain all of the rinsewater out of the lungs etc without having to take everything fully apart between diving days.
. You don't want to mess around with rechargeable batteries or open parts of the unit that are not meant to be frequently opened.
