I’m a MBP+external screen (docking station) guy so I totally get the portable Mac idea. But I would caution against using a MBA as a main machine for more than Office and web. It’s a sweet machine at a good price no doubt, but it does have its limitations.
So if the current 16" MBP plus an external screen (wasn’t there something like TDM to use an iMac as an external display?) isn’t your thing, the portable Mac route is basically blocked since I wouldn’t buy a 13" MBP so shortly before the widely expected refresh.
I wouldn’t like to have my main Mac serviced either, because I don’t want to be without it for several days. Plus, in your case it’s from 2014! C’mon man. You’re almost as bad as me with my 2013 MBP. :)
So if I were you (again assuming you don’t want a 16" MBP) I’d buy a nice and fast external SSD. Of course a good SATA SSD with a USB-C 3.1 dock would be sweet, but your iMac from 2014 doesn’t do that. Maybe TB2? Don’t now how economical that is. Whatever you choose, you’d be able to use this disk as a fast boot volume until the next iMac refresh. Then you buy that iMac and keep on using this nice SSD as external storage or backup or TM or whatever.