HR is genetic in the way it responds to the training and how low it'll get with how much volume. Just as woman naturally have a higher resting heart rate than men, about 10bpm, while being equally conditioned. Yeah it definitely responds to training no matter what, I wasn't saying that you naturally had that low a HR without training. If you did, you would have bradycardia.
Sounds like you would benefit from the sprint training more than the LISS at this moment yes, not that you should cut the longer sessions out completely. I would tone them down to 2 times a week and then do sprints 2-3 times a week for about 3-4 weeks, take a break for a week or two and then see where you are at. Imo, increasing sprint volume should be done in blocks as they are very demanding. Especially with all the other training you're doing. There are plenty of sprint/HIT protocols out there you could follow, I would start conservatively and then build up the number of sets gradually in the 3-4 weeks.
I think
@biscuitsbrah had a pretty interesting one from Joel Jamieson?
Is your best 5k time 22:30? If so, yeah then you would probably benefit from the speedwork, considering the aerobic base you have.
Btw, going all out, I mean, sparring is just as much about learning how to be efficient with your energy, but you probably already know that.