Reinstall doesn’t use a bootloader as such. All you need is a host computer that has a working uuu command and the needed files on said host.
If you want to image your phone as it is now then refer: Possible to make/save Librem 5 distro + data as an image file? i.e. basically boot Jumpdrive on the phone, and for that the requirements are the same as in the previous paragraph (but the needed files are different).
As you say, a USB-C dock (hub) is a relatively expensive way of connecting the phone to a host computer that only has a USB-A port. A USB-A to USB-C cable is one approach. A USB-A to USB-C adapter is another. (I use the latter as my host computer has no USB-C ports. Such an adapter is cheap as …)
As for booting the phone directly off a storage medium attached to the USB-C port … I’m not sure whether that can be done as yet but recent steps are heading in that direction. If and when it works, you would still need to contend with USB-A v. USB-C, but in that case an adapter would be the other way around. (I have plenty of USB-C storage and it works fine direct in the bottom of the phone, just not yet(?) for booting.)