Honestly, you're going to encounter unexpected issues no matter what dog you go from, and whether you adopt or go to a breeder for a lil' pup. Breed type will give you a general idea of your dog's underlying drives (for instance - whether they are scent-driven, if they're likely to be friendly towards kids, etc.), but it really doesn't guarantee anything.
I walk a Shih Tzu mix and he's such a dude. I love him. He's gentle, loving, affectionate, but with a big lion personality. He's not yappy at all. Alert barks a bit at home, can be a bit barky with certain men or things he's unsure of, but generally a fun, playful, happy, sweet little guy.
I also used to have a Lhasa Apso x Maltese mix, who was awesome. Grumpy old man personality, but so loving and loyal too.
There are so many young dogs coming into shelters right now purely because people purchased them during lockdown and their circumstances have now changed. I totally get your reservations (I had the same worries before adopting my dog), though. But the benefit of adopting a teenage/adult dog would be that you can see what that dog's personality is like and skip the housetraining part. It's really worth a look. Especially since you're not 100% sure what breed to get. Mixed breeds can be awesome (and I personally prefer them, even though my dog is purebred!)