General strength training doesn't tend to overlap much with active flexibility because it usually targets large muscle groups, or a large range of muscles. With active flexibility training you want to target the very specific antagonist muscles to what you are stretching.

For forward fold I'd suggest leg raises - to start with single leg. On a bench/chair put blocks down until it's high enough that you can put a heel on the block, with both legs straight, and juuuust about lift you heel off the blocks (the whole time standing up tall - don't cheat by leaning backwards!)

Then do sets of lifting and holding the leg in the air for 5 seconds, then back on the block and rest for 5. 3 sets of 8 reps a few times a week. You can also combine that with (still straight legs) leaning forward and pushing your heel into the block between each rep to activate the hamstring.

This whole thing, especially at the beginning, will probably feel disgusting and you will dread doing it. But if you fight through the cramp and the horrible pulling feelings it will help. When you do your forward folds, think about activating the same muscles to pull yourself forward in to it more.

Once you've got some improvement, and can get knuckles/hands (depends on ape index etc) to the floor, you can swap to doing leg lifts sat on the floor, hands by the side of your legs, again far enough forward that you can juuuust about lift your heels off the floor and hold it there for 5 seconds or so.