As a "tactical athlete" (SWAT) over the years I've found my ideal balance:
1. A framework of submaximal barbell work, 2 to 3 days a week
2. Kettlebell finishers for strength-endurance, conditioning, and that WTH effect
3. Roughly 80% LISS and 20% HIIT for cardio conditioning
Barbells are my cornerstone for strength. Simply put, no other tool gives me as much bang for my buck for limit-strength. I use a 3 day Tactical barbell template, it suits me perfectly in terms of staying sub-maximal and manageable while improving strength significantly on a long term basis.
I keep the kettlebell work simple...5-10 x 10 swings as a finisher 2-3 x a week, immediately after a weight workout. More or less depending on how I feel and if I have anything strenuous on my plate for the week. Kettlebells are a staple in my conditioning work too, when I do HIIT style training. Sprints or hill sprints mixed with swings or snatches add an entirely new dimension to cardio and overall conditioning. The bells take care of my grip, I find I don't need any extra/specialized grip work.
One or two 6-8 week aerobic base building blocks a year. Maff-style running along with bodyweight/calisthenics and kettlebell circuits...no barbells. As a matter of fact, this year I'll be doing Base with S&S + Pull-ups.
After Base, a regular week of cardio for me looks like 2-3 Maff runs or 2 Maff runs and 1 HIIT.
I don't really have to change my training much come PFT. I use weighted-pull-ups in my strength work, and my cardio is taken care of year round. Those are the two toughest bench marks in our unit as far as PFT is concerned (total pull-ups, 1.5 and 6 mile). Push-ups stay relatively constant year round with very little training, so as PFT approaches I might add in a few sets every week or not.
By far my biggest lesson over the years has been to stay submaximal and consistent. I'm not sure who said it, but "stimulate, don't annihilate". That being said, it's a fine balance staying submaximal but still doing enough to improve. I fell into both traps; going too hard, balls-to-the-wall and a little too "submaximal". The hardcore approach just isn't sustainable for me long term...I can see it's use for selection prep or something similar though. Going "too submaximal" felt good, but honestly as a "tactical athlete" my times and fitness markers suffered. As always, it seems the answer is somewhere in the middle.