I would suggest no injury time being declared just 90mins with the stadium clock being stopped for subs, injuries, etc. No adding 30 seconds per sub nonsense. Just stop the clock for as long as it takes to do the sub. If the team takes 5 minutes doing a sub then the clock is stopped for 5 mins.
There are valid time wasting techniques (e.g. Kane/hoof/offside/spit/genius
) and illegal ones. Illegal ones should be punished by adding a minute back onto the time remaining even if the time wasted is less. That would discourage time wasting fairly quickly.
I'd take the time keeping away from the ref too as he has enough on his plate.