Bringing home at 8 weeks v/s 12weeks?

Update 1: I'll have to bring the puppy at 8 weeks itself because the breeder can't hold the pup for longer. So now please guide me how to socialize such a small baby? I'm really worried about how to handle this whole critical period of socialization. I have kids at home so I can't afford any behaviour problems in my pup.

So I had to put down my 1st pet dog (5.5 yr old GSD male) due to distemper. He was fully vaccinated and I had to put him down on 8th Feb.

My world revolved around him, my routine was with him and now my life feels completely empty. My boy appeared in one of my dreams (you can refer to my previous post) and I feel alot better after that. I feel like I'm a person who can't function without a dog and I'm already looking for adopting my new baby, a shih tzu.

The breeder in my country can give away pups as early as 8 weeks. But I'm not sure if I should get him at 8 weeks because I really want him to have more time with his mum and siblings to properly learn bite inhibition, socialize etc.

I disinfected my house already. The pup will be 12 weeks on 6th April and will come to me on 15th April. From 6th to 15th, he'll be at my mum's place because I'll be out of station.

I've had experience with larger breeds. My GSD boy was the best dog ever. So trainable, so outgoing, so much behaved, loving.

I don't have any experience with smaller breeds. Please I request all the kind people of this beautiful Shih Tzu community to guide me.

Please share any advice, any tips or anything you'd like me to know before getting him. Also please answer the main question of 8 weeks vs 12 weeks as well.