Hiya guru tyoe people

We took Dizzy to her first socialisation class today. I'm very happy with it and how the boss type bird ran it, really pleased (ran under Attila Szkukalek).
Small class of 6 puppies.
2 labs (confident puppys), 2 Yorkshire Terriors(one nervous the other petrified) and a miniture Duchsun (gorgous happy lil thing).
She places Dizzy being quiet and reserved with the other smaller dogs to meet eachother since the labs were very outgoing boisterous mokeys and wanted to keep it positive for the more nervous puppies.I'm more than happy with how she did it.
Dizzy didnt whine or shake, just sat between our chairs, head in paws, back legs stretched out behind her and watched the world go by.
.
But I wanted to ask, Dizzy was very reluctant to meet the other puppies and people (it never occured to me that she wouldnt want to play or investigate). I didnt push it and let her do whatever in her own time. Towards the end of the class she did stand up and take some interest in the petrified Yorkie as it went to see the trainers calm adult dog but that was it.
As her jabs arnt finished yet we've been carrying her to different places to meet new people in passing and see the sights. However if a stranger offers her a treat (at our request) she isnt interested.
She doesnt whimper, whine or hide just looks bemused.
Had a few freinds round and in her own time she'll come forward to play tug if they instigate it but it can take up to 1/2 hour for her to come out of her shell.
I always thought puppies were food/fun seeking missiles!
She came from a litter of 9 pups and was easily the most llaid back one. Not nervous since she would be at the front to say hello and took to us quickly.
If one of the other puppys kept giving her greif she'd shrug them off, barely noticing them and then after a fair while sit on them before fighting her way to the front again.

.
How do I not only socialise this kind of puppy in the most positve way given that she wont except food from strangers and takes such a long time for her to get used to people (ya know when your out and about, you cant ask someone to spend half an hour for your
puppy to get used to them)or will this come with time and constant socialisation?
Also, I'm going to take her into the front garden tomorrow to get used to the sounds of cars and people AND a different area since I noticed today when I tried to walk her to our car, she just couldnt make herself move off the front step, it was just too much.
She's going to be one of those dogs that you see people DRAG on their walk, she's so reluctant.
I do think she's pretty laid back, but I DO know that she's worried about strange people/places and easily spooked.
How do I get the best out of socialising her and maximising on it?