There are no options checking all the items in your list, you'll have to sacrifice some of them if you want the device now. The requirements that are most difficult to fulfill are hackability, full openness (even the Pinephone doesn't fully comply, as detailed by Preflex) and waiting/shipping time.

The Pinephone is a very nice device with a lot of hackability and OSes will improve, but so far they still need users to be quite flexible as to what they can do and to which level they are prepared to skew their usecase to adapt to current limitations. It's cheap and will ship every few weeks, reliably.

The Pro1 is on the way to mainline and its form factor means there's a niche community behind it to port OSes and support it on the long term, pretty much like the N900. It's hackable with open bootloader and one dev is close to achieving multiboot on it. It's significantly more powerful than the Pinephone and smaller in height and width, just a bit thicker. Its keyboard and performance make it a good device to run desktop Linux in chroot or LXC. Progress is being done towards mainlining on this SOC so Leste and pmOS may happen in the future, but that will require some patience; until then current feature complete Linux OSes include Sailfish and Ubuntu (not sure GPS works on Ubuntu though). It's the closest to the N900 and has some repairability, but ships in 3 to 4 months, so you'd need another phone in the mean time. And yes, the Pro1x is a crowdfunding device, but I'd still recommend it over the Pro1 from the store because lead time for the latter is not really known. The company has a good track record and they have already produced the same phone (Pro1) in the past, so the main risk is delays (a real risk since they do "first come first serve" and there are 1000 buyers in queue), but not cancellation. But waiting until March may be unacceptable in your situation. If you can find a used Pro1, that might work, but I'm afraid most of them would be on eBay so that's a no go. Now may be a good time to check for used ones on the forum though, since some users may sell their Pro1 (still powerful enough) soon to upgrade to the Pro1x. I'm currently uploading a Ubuntu Touch vs SailfishOS video for this device, should be available later today. [Edit] Here: https://youtu.be/rgxrJj37xOs.

Xperia X and variants running Sailfish, but this will again come with some trade-offs regarding your list. They can ship whenever you can find a used or reconditionned Xperia; there may still be some new units in online stores too.