Sorry for the delay coming back to this thread, and thank you to everyone who's posted.
@Veterinari, that's encouraging about second socialisation periods - thank you. Also useful to read about trigger stacking (and thanks again @Wolfiefan). I'm glad you say that about neutering. I read some behavioural research and came to the same conclusion - she's not being neutered till after a season or two for this reason. In fact, she's in season now (which may explain some of her recent jumpiness??). I'd also read that research about behavioural issues being the leading cause of death - it's sad. It's frustrating that the guidelines were so completely physical health focused around the time of first lockdown. Our vet told us that they couldn't vaccinate, and that, until they could, we shouldn't even allow our pup in the garden because we have a lot of foxes (we flouted this in the end as it was unworkable).
@Madbengalmum, that sounds tricky, and must be frustrating, especially when you're so experienced, to see what lockdown can do. Thank you very much for posting.
@Sitdowncupoftea, thanks so much for posting. I hadn't really thought much about muzzling. We have a "nervous" bandana that we can put on her, and we're not taking her to places with dogs off-lead at the moment (she's in season). She's a small breed (miniature poodle), so looks fluffy and friendly, and it's hard to make sure people know not to just try to stroke her, but I hadn't thought about muzzling. Will read up about it. The person we got in was a qualified behaviourist (reputable organisation). We mostly focused on issues in the house at that point, which have mostly resolved now, but also did get the distraction advice about how to manage behaviour with other dogs. Perhaps we'll book another session outdoors if we can.
Your posts have given me extra impetus to make sure she goes for a walk with the calm and lovely dog of a friend at least twice a week (and me and the friend). She does bark frantically at the dog when we first meet them, but now quite quickly starts walking calmly beside her. Any frantic movements of the other dog, or any playfulness, and she barks again, but yesterday there was almost the start of calm/friendly bum-sniffing(!), so possibly some progress.
She's also getting a little better with cars (not jumping and trying to run after them) over time. Other people, she walks past quite well now (unless they try to stroke/approach her, in which case she cowers), unless they're on scooters/bikes/running very fast close to her (in which case she barks/strains).
Thanks again to all. Very grateful for any further thoughts/general company with this, too.