Using any plumbing fixture- flushing toilet- running washing machine causes water closet in the 1/2 bath to gurgle. This toilet also
Doesn't flush properly. I think it's a vent problem but short of busting out some concrete how do I vent this toilet?

I think it's a vent problem ?
You are correct! You have a blocked vent. The gurgle you hear is the system attempting to vent through the trap water seals.

short of busting out some concrete how do I vent this toilet Your toilet is already vented, (see image).
The solution would be to snake the lavatory roof vent. Put out enough snake to reach the base and 18 feet more. Good luck, nTom