Look at the ratings of the things you want to run simultaneously. Only then can you make a logical assessment. 1/2" supply shouldn't be asked to source more than about 5gpm. if you want more than that, then you probably want a 3/4" valve with associated supply pipes. It also depends on the temperature you keep the indirect. The hotter it is, the less hot you use since it is mixed with more cold. Then, decide how long you would run all of those things at once. How big is the boiler? Is the indirect set up as a priority zone?
I think you'd find it cheaper to replace the indirect with a larger one, if you end up needing it than a tankless.