If you read the first article I attached, it has three options: bike, rower, and treadmill. A good option, like I said, is doing each one once a week. This adds up to three hard(er) conditioning sessions a week, which is what you want.
I’d just try that out for a few weeks. Then you’ll know if it works for you, and how you like it.
Do that stuff (the harder stuff) then do some band pull-aparts (like 100-200 reps), some stretching/mobility, and go for a walk if you just want to be at the gym longer.
Like I said, walking is awesome and very overlooked because it’s not “intense”.
Some pull-aparts will take nothing out of you for your big lifting days, but will just add in some upper back/rear delt work that we all need.
Stretching/mobility is needed in everyone, especially in the hips and shoulders. Rotating between sitting in a “third world” squat and doing some shoulder dislocates a few times would suffice, or you could do a specific sequence of stuff, like DeFranco’s Agile 8.
I think if you want to do any assistance work on your off days, I’d limit it to chinups and pushups. No need to lift weights, then it’s not an off day.