If that's the case, then well, control strips AND densitometer together will only reveal the problems (not telling what's the actual cause is) but there's nothing you can do, but just to be more careful next time. And, you cannot see the problems before it's too late. The next time is a new batch so the results are useless, or they tell you only one thing: be more careful.

And again, you don't need a densitometer to be extra careful.

Only thing you can do with test strips & densitometer with one-shot development, is to find the correct time for your processing style (including agitation, heat losses etc -- everything that remains constant). Then you just maintain the process, and you can check it once in a while. But, it does not help random problems at all as you cannot take a corrective action like with continuous processors.

If you want to check for chemistry mixing error, buy a precise pH meter.

But, it's not that difficult. Just follow the instructions and your results should be very good. Probably better than from most commercial labs.