I dont think you can use a plug in a tire with a tube. IDK for sure, but wouldnt that Lump of the plug be a stressor on the tube? Besides, a plug is to keep air in,, & thats what the tube does. No plug..
If you want to do the work yourself, then maybe it makes sense ( dollars & cents, sense) but if that tire is sitting at 2/3 tread wear & its gonna cost you $30.00 labor, I'd say thats not the best deal for you. If you want to risk using a patched tube & do the work, then just be aware, patches dont always work out & a flat on the rear is no fun.

Others do their own tires & Ive given it a shot. The front isnt too bad, but if you dont know what you are doing, the rear is an absolute BEAR.
I recommend you buy a tire & tube & have someone install same. Next time, maybe you should give it a shot, but hang out & read & learn some more before you decide.
Call around & get some tire costs & installation costs. Some will go easy on installation if you buy the tire from them. TRy to make a deal.

I agree