The kits with a Bigsby-style trem in on hollow bodies often come with the wrong screw to hold the trem to the top of the guitar. Three of those four long screws that are in the compartment with the bridge post inserts should do for holding the trem to the rear of the guitar (if they fit). What you’ll need to get is a couple of much shorter screws that are fully threaded to hold the trem in place to the top. The screw will just pass through the ply top, there won’t be a thicker block under it, so you don’t want a screw with a solid section at the top as it won’t grip the wood at all. You’ll want ones that are at least 5mm longer than the trem+ply thickness so that all the screw length in the ply is full-width, so 15-20mm should do. Chose a head shape that you think will best fit the casting, though a countersunk design should work.

Though I’ve never used one, I’ve seen some similar Bigsby designs don’t have any screws through the top, it’s held in place by string tension alone, plus the end screws. So those screws through the top probably don’t have to work too hard.

With the deep body, fitting the strap button through the central tailpiece hole would put it significantly off-centre. See what you think with the trem on the guitar, but I’d tend towards keeping the strap button separate.

A lot of the kit screws aren’t of the best quality, so you may be better off sourcing your own good replacements from somewhere like Allparts or WD Music. The small tuner anti-rotation screws are notorious for being cheap alloy and have a habit of shearing off when being inserted, especially when into hard maple (though the kit tuners themselves are pretty decent). So always drill decent pilot holes and wax the threads (this goes for any screw regardless of quality) to minimise this risk.

Be sure you know how you’d wire up a piezo bridge system, especially an active one, as it’s not something that you can easily wire up outside the guitar and fit through the pickup holes like the normal wiring harness. You’ll need to be able to bring any extra wires out through a pickup hole and solder to the harness just before insertion, including any battery box wires (if you don’t keep it as a separate system with its own jack).

With dog-ears, you don’t have any height adjustment at all, so if the pickups are too far from the strings, then you’ll need to fit shims. You can buy them, but I’ve made my own from fibreboard. But as the shims don’t have a gap round the edge, the pickup wire needs to be led through the hole in them before soldered to the harness. So that’s something that needs checking and sorting before the harness goes in. Which means that you’ll really want to set up the action first to check the pickup height against. It’s well worth writing out a checklist/order for final assembly to make sure you don’t leave anything out, requiring a removal of the harness to add something you’ve missed e.g. a bridge or a tailpiece grounding wire.

Soundholes are primarily to ket bass frequencies escape from the body. The bigger the soundhole area, the more bass you get (or so I’ve read). So to get a balanced pure acoustic output, you’ll need quite a large hole (or series of holes) which will get the top resinating to the bass frequencies and so increase the risk of feedback at higher playing volumes.