Don't get caught up in HIT and HVT. They are just labels. HVT generally describes anything that isn't strict HIT. It doesn't necessarily mean that it is horrendously high volume and frequency. It may be, but it may just be slightly higher volume.

My suggestion is this.

Keep whatever HIT routines you were using. But limit yourself to two sets per exercise instead of one.

Might not sound like much, but keeping the same routines, you've just doubled your volume.

Whatever you do, don't be tempted to go back to doing 3 or 4 sets per exercise, with the same voume of exercises per workout. This will just tempt you to back off on the intensity to the point where it is pathetic.

The logical increase in volume should just be adding one more set to each exercise. You should still try and push yourself as hard as you can, but in the confines of two sets rather than one. Stick with the slower reps and try to work to failure, but don't worry if you can't push all the way to failure, that is what the second set is there for > > so that you don't feel guilty for not having done enough, and have another chance to push yourself hard.

Remember, the line between HIT and HVT, if you have to use those labels, is crossed when one starts backing off on the level of effort in order to do more, giving volume priority over intensity. Just don't go too far.

Think of intensity as your orange squash or your ribena, think of water as the volume. Add too much water and the ribena becomes so weak you can barely taste it. That is not what you want to happen. Then you'll end up doing about 30 sets, 4 days a week and going nowhere.

I have a theory that a person can only do justice to a maximum of 20 total sets per workout, regardless of the number of exercises or sets per exercise.

So clearly, more sets per exercise means fewer exercises per workout.

So you could have 5 sets of 4 exercises, or 4 sets of 5 exercises, or 2 sets of 10 exercises, or 1 set of 20 exercises etc etc. Or a combination of that. Just don't exceed 20 total sets per workout. That includes warm up and work sets.

I used to find that 3 sets of 4 exercises worked well. But 3 of them have to compound e.g. deadlift, dip, row or squat, bench press, pulldown.

Basically, do not jump to the polar opposite of HIT. Instead, gradually move away from HIT until you find the balance of volume and intensity that is appropriate for you.