Generally speaking, I hate flexis. It’s not because the tool is inherently bad, it’s because people suck at training their dogs.
My sheltie has a flexi and a 6’ leather leash. I taught him “criteria” for walking…
Criteria 1 - if there’s a loop in the leash, it is his job to walk politely either next to me or slightly behind. If he hits the end, he must return to me, sit at my side, and look at me. A looped leash might be the leather leash, or it might be the flexi “locked” to a short-ish length.
Criteria - if the leash has passive, following pressure (i.e. a typical flexi), he may walk wherever he wants while it is still following. If he hits the end of the leash and the pressure is no longer following him (either because I locked it or he went too far), he must stop what he is doing and return to a sit at my side.
Most people use flexis as a quick-fix for making a dog more tolerable despite not knowing how to walk on a leash. This is why flexis get a bad rap… because many dogs on flexis are out-of-control lunatics with no obedience training (and possibly an owner to believes it should be treated like a human child in a furry suit, not an animal). Not ALL dogs on flexis are terrible, obviously, but it does seem to be an unsettling proportion.
OP, your best friend is going to be consistency. NEVER GIVE IN TO PULLING. Start practicing in your living room. Sit a smelly, tasty treat on the floor and hold your dog on a leash on the other side of the room. Take a step toward the treat. If the dog doesn’t pull, great, take another step. If he does pull, stop and take a step backward. Make it clear that the ONLY way he can get to the treat is if you take him there… and that if he pulls, it is counterproductive for where he wants to go.
When he does this perfectly in the living room, branch out to the backyard. Then to the sidewalk in front of the house. Try it with people he wants to greet and things he wants to smell in lieu of treats.
If he doesn’t care about food on walks, have him skip a meal and take him out with something super smelly and tasty like liver. If he is hungry enough and you have something that is tasty enough, he will pay attention to food.
OR, use approach to anything he wants (including smelling something) as his reward. That will make it easy for you, as you let him choose his most valued reward.
You may have to take a period of 2-3 weeks where you never get to exercise him on a leash because you can’t move more than 10’, so it would behoove you to think of an alternate way to exercise him off leash while you are putting basic leash skills on him. This is where many people falter, I think.
If pulling is never ever rewarding, and if pulling ALWAYS results in retreat from what he is trying to get to, he will stop pulling.