OK, you currently have two fairly recent computer (Air, & Mac Pro). You use the Air for travel as needed. For your stated uses both computer can do everything you need.
Given that I would setup the Mac Pro as a desktop computer with a
Thunderbolt hub ( I use both a Blekin, and OWC but would get the Caldigit now); use a
dual monitor arm mount and get two 4k curved vest mount monitors (curved is a preference as it reduces glare in my case, flat screens are fine as well) as big as I can fit. Saw you only have room for a 24" but I am assuming the monitor will address the space issue. If not one is fine. In terms of file access I would use iCloud (manually) to share files across computers. By manual I would create a folder on iCloud that I copy which files I want to share as opposed to defaulting all files to be stored on iCloud.
All of the desktop kit would be able to be used with you next purchase of a new computer (unless they change connection ports), but right now I would not get a new computer as the ones you have should last you another 5-10 years.