Walking issues with 3 year old dog

I have a three-year-old intact male sheepadoodle. I got him when he was three months old as a rescue. He’s always been anxious, but generally happy and friendly. I’ve worked with him on walking since the day I got him, and things improved for a while but within the last couple months I’m having a ton of trouble with him on our walks. He is a relentlessly frantic sniffer and marker. He will drag me down to get to a smell, and double down on sniffing it once he gets to it. We have snow on the ground now and I can see what he’s sniffing – evidence of other dogs: pee, footprints, feces. I’ve tried having more structured walks and having time for him to sniff, but nothing really seems to work. And it doesn’t matter how long we’re out, he never ever gets tired. In fact, he seems to only get more frantic as the walk goes on. He knows heel and will do it, but only for a few seconds before he gets distracted by a smell. It’s like he has dog ADHD or OCD. He is terrified of a harness, I’ve spent hundreds and hundreds of dollars on different kinds and did all kinds of desensitization training with them but he runs away every time I have one in my hand, so I just walk him on a flat collar and end up pulling him.

I should note that he’s not a good dog greeter either, he is a very anxious greeter. He will whine and pull HARD to greet another dog on a leash, and off-leash he will run up to any dog. Both instances look like he wants to get at them to play. But once he greets them he has a pretty stiff, flagging tail and doesn’t really know what to do. With friendly dogs, he’ll play; with overly rambunctious dogs I can tell he gets nervous and doesn’t know how to not engage with them so he matches their energy; with dogs who want to fight him, he won’t really back down and has gotten into scuffles. He rarely, if ever, just ignores a dog.

I’ve had him evaluated with a behaviorist and they determined that he’s not aggressive but he’s very anxious due to genetics and he might just be an on-leash dog forever. I’m feeling very frustrated because I can’t give him enough exercise on a leash (I would love to keep taking him to our local trail system where people walk dogs off leash but as mentioned above he always seems to have some kind of issue there) and walking him on a leash is so unpleasant, for both of us. Any advice much appreciated.