I would not solder the joint after PEX has been connected. If you need the connection to be farther out, then solder on some more copper to where you "can" use the crimper.
I'm adding a wet bar in a finished basement and need to run pex to it (finished ceiling, limited access, etc). I have a few options I have looked at and discarded and am now down to the short straws. I would prefer to make the connection to the existing copper water lines in what is a fairly small opening in the ceiling. Big enough to cut in, solder in, but not to crimp/compress in, so here is the question. Can I attach and crimp the Pex to the t-connector and then solder the t-connector to the copper last? Of course my preference and the right way to do this would be to crimp last, so no need to tell me that. I have looked at and thought about products like Shark Bite, but due to location (closed, in basement ceiling) I don't want to risk a leak down the line, so thanks but no thanks. Yes, I could run the line back to an open part of the basement, adding 20' to the run, but that is my second choice. I'm just wondering if there is any real risk to the crimped Pex connection if the sweating to copper is done last. Keep in mind this is a T (copper-Pex-copper) connector and not a straight copper-Pex adapter.
I realize you dismissed the SharkBite but I would say take a second look. From what I have seen they are fast, easy and best of all solid. Perhaps someone has seen them fail other than operator error but I am not aware of any.
With Uphonor pex, you use an expansion tool. You could have the transition connection soldered onto the copper, have the pex pulled down where you could reach it, expand the tubing and then slide it over the barbs on the fitting, and it just collapses to make the connection - you don't need to have the tool around the tubing in place like you do with a crimp system. Now, you don't have forever to slide it over the fitting, but it does take a few seconds to collapse. If you fail, you'd have to cut the tubing off and try again, being faster this time, but it does work.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014
There is a crimping tool made for tight spaces. It uses a hex nut to tighten down the crimp ring. It's a slow process but in your situation it would do the job.
It depends on where the access hole is relative to the connection, but a "cinch" tool does not go around the tubing, it grabs the "ear" of the cinch and squeezes it. soldering after the PEX is connected, unless it is a very long piece of copper will affect the joint.
Licensed residential and commercial plumber