Hi,
I work for a rehoming charity and have fostered three chihuahuas over the last year, each for approx a month. I wasn't overly keen on chihuahuas until I had the opportunity to really get to know some and they are cracking little dogs with hilarious personalities. I would definitely consider one when we do get a permanent dog.
I think that the breed stereotypes can become a self-fulfilling prophecy because owners think that's how they are meant to act so put less effort into training otherwise. (e.g., if they are hard to toilet train I will just accept they are never going to be fully toilet trained. Or, small dogs are yappy, i wont bother to train mine to stop barking).
I currently have a pair (3 years and 5 years) staying with me. They were from a hoarders house, the owner was unwell and they never got walked or taken out in the garden. The only time they are vocal on walks is when they see another dog close up but this is due to the lack of socialisation and this has already been improving over the last month (with rewarding them for being quiet whilst a dog is at a distance). These girls don't bark any more often than other dogs we've fostered but it is surprisingly loud and shrill when they do bark which gives them the yappy reputation.
I would call them anything but hyper! I'm working from home and they sleep/snooze under a blanket in their beds (or on my lap) most of the day. They will have little short play sessions with one another but only for 5 to 10 mins every few hours. They have also started playing with toys, which is adorable, and they will play for again maybe 10 mins a couple of times a day and then they are zonked.
They weren't toilet trained when they arrived but with regular opportunities to go outside and treats when they toilet outdoors they are already making an effort to hold it.
The other one we fostered was a puppy (and I was very very tempted to keep her, if we'd seen covid coming then we probably would have).
She was an incredibly affectionate, bright and gentle puppy. We put loads of effort into socialising her and starting her training whilst we had her and she was super bright and easy to train. She was not at all yappy, in fact she only made any vocalisations when playing. And even as a three month puppy I wouldn't have said she was hyper, she would have zoomies a couple of times a day like all puppies.
So what I'm saying is, don't rule them out because of concerns about them being hyper, they don't need much exercise compared to other breeds so as long as they are getting what they need and some play sessions they shouldn't be constantly hyper and demanding activity.
And they are not difficult to train, it's just knowing how to do it. Dogs Trust have got lots of advice for training all the basics you might need for whatever breed you do decide on :)
www.dogstrust.org.uk/help-advice/training/
Best of luck :D