I just go for total reps and don't really do sets. This might be more stressful for your body the way I do it, but it is easier in my opinion and saves more time. Typically what I will have people do is 30-50 total reps during 15s, 20-30 reps during 10s and then 15-20 reps during the 5s. Most of the people I train with seem to really like this setup but I have noticed that it can wreck your immune system if you do not get adequate nutrition or sleep, but lifting in general can do that if you aren't eating/sleeping enough, so...
Anyway, since it might not be immediately obvious how you would do this unless you've ever done DC or Myoreps, I will explain. Usually it takes the people I work with a session or two to wrap their minds around how this works.
During 15s, doing flat bench - rep goal is between 30 and 50 reps. John Doe does his warmups then gets ready to do his working set, which is using 185 lbs. He gets set up and begins, doing 17 reps his first shot - I always have them stop 1-2 reps away from failure. You can tell when they are getting close to failure because the bar slows down quite a bit. So anyway, he racks the bar, I have him take 15 deep breaths - usually this is 30-45 seconds - then once he is ready, he begins again. This time he gets 10 reps before he stops - rests for another 15 deep breaths then starts again - gets 6 more reps. I have him stop and I note it like this: 185 lbs x 17, 10, 6
I only ever have them do it in three separate attempts.
This might be something you might like to try out. You do get through your sets a lot faster than you would using conventional sets. I will warn you though that during the 5s, you will definitely need longer than 30-45 seconds between attempts. I let them do 20-25 deep breaths during the 5s instead of 15 deep breaths. Yes, this keeps them closer to failure than using standard sets x reps.
Why not keep volume constant? Well I ask instead, why keep volume constant? There isn't a lot of reason to keep volume constant. We are trying to continually add weight to the bar and become stronger over time. It is very difficult to keep adding weight to the bar if you are obsessed with keeping the total number of reps the same. Besides which, as your load goes up, so does the workload, so even if you drop volume your workload is still going up.