I've been lifting 5 to 6 days a week for nearly 20 years. Now that I'm in my late 30s, my focus has shifted from gaining mass to more of a balanced and longevity focused approach. I still want to lift 3-4 days a week and would ideally maintain what I have to an extent, but would use the remaining 2-3 days of the week to focus more on zone 2 cardio days and or short HIIT training sessions. Are there any solid 3 or even 4 day training splits you all would recommend?

5/3/1 is a good fit - part of the program is having some light conditioning and hard conditioning days.

I lift 4 days a week, walk with a weighted vest for 3, and do sled stuff 1 or 2 days a week.