Off-season is your opportunity to get better. You would not want to mix things up during the ramp up to your competition. But your competition is over. Now is your opportunity to knock off the rust and improve upon possible weak link(s) that have built up over your season (race or races).
Has your mileage taken a toll on any of your joints? Your ankle, knee, hip, low back, shoulder? Now is a perfect opportunity to strengthen/prehab/rehab whichever area is a weak link. Before it becomes an injury and sidelines your fun.
Do your movements look/feel natural (or) forced? Work with your coach on your technique: stride length, running technique, swimming mechanics, and I'm sure your running/tri coach will have other fine tuning details for you.
For opportunities 1 & 2 working with someone specifically that can address your specific concerns and areas to improve makes the most sense. You want your homework/program to be individualized. Of course for the 3rd opportunity, Strength, a specific program would deliver the best return on investment (your time). But you can still improve from a generalized program.
The stronger you are the faster you will potentially be. But let's clarify - you aren't a bodybuilder.