There are several blackberry keyboard projects out there. I've been on a similar journey to OP for a while. Currently I've got a phone I'm fiddling with putting a jetson xavier inside a rotary phone and another based off the featherwing with a blackberry keyboard and touch display and a feather compatible linux SOC (GiantBoard).
I recently got a ClockworkPi DevTerm and have been so pleased with it that I'm seriously considering their GameShell as a phone platform. It's modularity is great.
I'm so excited that other people are working on DIY "phones". I personally think we are going to see this scene keep growing and phones will finally be more like the custom computer market very soon. The fact that people are finally putting linux on apple devices is also very very encouraging. There are so many bricked iphones and ipads floating around with great displays and sensors that are totally useless because we can't run a reasonable OS.
The connector is extremely hard to solder. They are over $1 each and so far I have destroyed 7 already.
I have tried via hot air, heated plate to hand soldering with the iron. The hot air will melt the connector so that won't work unless you heat from the bottom the the PCB. Hand soldering is also almost impossible because you can't really get to the pins and if you just slightly touch the plastic it will melt immediately. I also could not get it to work with solder paste and a hot plate. The pins are so small they don't want to adhere to the PCB. I may need to get gold plates pcbs and a stencil to place the exact amount of paste I need to get this to work. A QFN package is easy to solder in comparison.
You almost definitely would benefit from getting a stencil. Instead of a hot plate you could do a toaster oven. I got an "air frier" convection toaster oven at walmart for $50 open box. It's perfect for reflow. Also take one that you've already ruined and test the melt temp on the plastic them put a thermocouple probe on the plastic.
Is there any particular reason you can't use the featherwing package that adafruit sells? I've seen a few other folks on tindie selling other board and screen combos but personally, for my money, the feather format is pretty good for almost anything I could want to do in this form factor.
I do too, but I really don't miss the tiny, non-tactile screen. A modern Linux smartphone with the form factor of the Nokia E90 Communicator or the N900 would make me ecstatic, though.
https://en.wikipedia.org/wiki/Nokia_E61