Struggling with walking my foster dog

My first dog died several months ago. My family decided we wanted another dog, but also wanted to make sure the new dog is a good fit. We found a small local rescue who had a dog that seemed to be a good fit. They offered that we could foster the dog as a prelude to adopting him if things work out. The dog was currently with a different foster home.

We connected with the other foster family to get info about the dog. He's a 5 year old, 50 pound hound mix, described as a couch potato. The only caution was that he was overweight since he wasn't able to be walked as much as needed. There are 3 of us, and with our previous dog we averaged 60-90 minutes of walking a day, so we figured that wouldn't be a problem.

Now what we weren't told was that this dog has significant reactivity, of the "frustrated greeter" type. Indoors, he is indeed a couch potato, completely perfect for us. But outside, he wants to greet every person or dog he sees, which leads to pulling and barking. The rescue did provide us with a session with a local trainer to address his reactivity and provide us with engage-disengage training strategies. But it's been nearly a month since the trainer, and while we've tried to apply what the trainer taught us, I'm just not seeing much improvement.

We don't have a yard, and live in a Seattle townhome development, so it's not quiet here. Quite a few other homes have dogs, and there are multiple delivery vehicles every day. If there's nobody around, he behaves himself. But if there are people are around, it's a crapshoot. If we catch things far enough in advance, we can redirect him. But there's a lot of corners that people can come around and surprise us, even at 8 or 9 at night.

For example, during this afternoon's walk, 3 children ran past us in a 2 minute window, which set him off for the rest of the walk. And then tonight, a guy walking his own dog followed us on 2 separate occasions despite my dog turning and barking, and me asking the guy to let us get away. So that was 2 walks in 1 day that ended with me desperately trying to lead my dog into the house. Walks with my previous dog were a fun adventure where we could go for a mile or 2. Walks with this foster dog are a stressful chore. A saving grace is that we've established that even just 20 minutes outdoors in a day is enough to prevent boredom-induced misbehavior.

I don't know the timeframe for a dog to learn disengagement and overcome their reactivity. And I do know that this rescue's policy is that if we decide we won't adopt, then we would continue to foster until an adopter is found, which we would be fine with. He's such a perfect dog indoors, that I really do want to make this work permanently. But I'm just very frustrated with where he's at right now. I'm open to any advice.