Hi, @TheRavenOfDiscord and welcome!
Excellent - another bass builder!
Although I am personally a guitar player more than a bass player, I have certainly built more basses than 6-string electrics and most of those have been through-necks. In fact, all of my full bass builds have been through necks - although I have made bolt-on neck replacements for 4, 5 and 6 string basses.
You talk about 30" being relatively rare. Well, certainly in the UK it used to be, but here there is a growing realisation within a lot of bass players that, nowadays, you can get just as good a sound from 30" basses, you can get great strings for them and they are MUCH easier to play than a 'standard' long scale. It is interesting that nowadays, I get more enquiries for short scale than I do for long scale...
There is a lot of wisdom already expressed in the above replies. Re-iterating some of those answers and adding a few of my own:
- The end of your fretboard is going to be around 23" from the nut, leaving you 7" for your pickups. That's plenty. Are you going Jazz pickups or P-type? or PJ?
- I build my guitar necks and bass necks to the same thickness. It usually ends up at around 21mm /22mm at the 1st fret (spine to top of fretboard) rising to around 23mm at the 12th. That is usually with a max thickness of 6mm fretboard, so it breaks down at 6mm fretboard + 11mm trussrod & trussrod cap slot which leaves me a minimum of 4mm/5mm timber underneath the trussrod
- I don't fit carbon fibre reinforcement rods for 4 or 5 string basses unless specifically asked. I do add rods for a 6 string bass
- I usually do a 3-laminate neck with the middle one being a 6mm splice. 5-piece laminates are great and I have done them at times, but I've never had a warp or twist issue with a 3-piece. However, I always use quality timber from trusted sources for the necks. I'm usually happy to use any structurally-sound timbers for the body!
- I fit a single, good quality 2-way trussrod. I personally think you can introduce more problems by fitting two trussrods than solve them. If the neck wood is straight, tight grained, properly seasoned and in the right grain orientation, and the fretboard likewise is a decent hardwood and flat, there should not be any warp issues in normal use and storage.
One last tip. With a through neck, remember that any neck angle needs to be built into the neck before the body is built onto it. Therefore you need to know what bridge you are going to use up front so that you can sort the geometry correctly. Personally, I physically get hold of the bridge first and check the ACTUAL adjustment ranges and then draw the nut/fretboard/bridge lines full size to work out the angle I need to build into the neck blank.
Very much looking forward to seeing your build develop!