I won’t speak on behalf of SixFab, but a custom board like that would likely be a lot of work, and cost for a rather niche use-case. Not that I don’t agree with you - I have had similar issue where the ports on boards are just in the wrong place and it make a case a nightmare - i used it as an excuse to buy a 3D printer and make my own cases (be careful it can be addictive)
I had neglected another option - if you are ok with using the main UART (which you apparently are!) you might be able to use CMUX - which creates virtual serial ports over the main physical UART. You need the driver on the Pi side, and also the modem needs to support it. I have had issues in the past with CMUX (you can find my post here: PPP, CMUX, ldattach on Raspberry Pi with M95) so you’re mileage may vary. I’m fortunate enough to have a Quectel LTE EVB with a BG95 and it appears to work fine on that, so in theory it should work.