Barking and reactivity in newly adopted pup, aka Am I screwing up my dog?

Hi! I would really love some advice (and encouragement). It might take a few paragraphs to get this out but here goes... I'm 31 and single, living with a roommate in a duplex apt. A month ago adopted a very large 9-month-old male pup (Great Dane/American Bulldog/??? mix). He had just been neutered the week before adoption. He is sweet and mellow most of the time, submissive toward people, and was totally chill for the first 2 weeks - he would come up to me or visitors and lie down belly up, he didn't bark much, and he didn't have a problem seeing other dogs when we were out walking (I walk him 3x a day).

His behavior has begun to change, and not for the better. He is still sweet and submissive to me and people, though he doesn't offer his belly so readily, which makes me a little sad! Worse, he barks (loudly, since he's a big dog) a lot at new noises/passersby (including my upstairs neighbors, who have been nice about it but I'm afraid are ready to kill me). I am trying to ignore the barking and teach him a "quiet" command, but when he barks in the middle of the night I just need him to shut up. I'm afraid if it lasts long, my neighbors will think he's mean, and worse might complain to my landlord, which could cause all sorts of new problems.

The biggest problem is that he is suddenly SUPER reactive to other dogs. I think that started after we were walking past a fenced-in yard down the street and a Doberman rushed out of nowhere aggressively barking his head off (and a few similar incidents - there are a lot of dogs in my neighborhood whose outdoor time is mostly in a yard or porch). He has gone up and sniffed a few dogs, but then when they make a movement that startles him, he starts his barking/lunging routine that is very hard to calm, and sounds/looks scary. I'm working really hard to turn away from other dogs we see on walks and training incompatible behaviors (sit/stay, watch me, etc) and I think it's getting a little better, slooowly. But since he didn't have this problem in the first place, I worry that I'm doing things that inadvertently CAUSE the behavior. And I'm nervous that he'll start being reactive to kids or others who do something sudden, though he's been fine so far. I'm sure my nervousness doesn't help, because he can probably tell I'm nervous when he meets kids, etc (I'm trying hard not to show it, but...).

Walking my dog is now incredibly stressful - it's the worst part(s) of my day. I'm working on all sorts of training, reading everything I can find ("Click to calm" is great!), and I'm going to have him evaluated by a trainer at the local SPCA this weekend to talk about next steps. I'm not going to give up, but I'm just really stressed and frustrated and nervous that he's not going to get over these new issues - not to mention that I gave them to him somehow! Thank you for any tips/advice/support!!