If you get a guard job then yes keep your civilian flying job as well. I recall a regional airline pilot washing out of upt back in my day so keep a back up. I know I wish I had kept my civilian job when I went to upt just for the senority issue.

I would not take a job to get "cargo" experience. However pic time is important, the larger the plane the better when considering civilian experience.

My main comment is to remember the world will be different in 6-10 years. 7 years ago UPS was a low tier airline and now it seems to be one of the darlings yet nothing much has changed. 10 years ago most folks would have chuckled if you said u where going to turn down United/Delta/AA etc and go sit on the FE seat at ups instead. By the time you have your quals and experience up who knows what the industry will look like.

Fly, learn, broaden your experience base, and enjoy the process.