Nitro is no more toxic than acrylic at the end of the day.
As for the yellowing, that takes quite a while to happen and in 99% of cases it wont make a lot of difference at the end of the day due to all the other ageing processes that happen

One of the issues with acrylic is it never really fully cures. This can lead to cracking, sinking, milky patches and not to mention not allowing the timber to breath a bit.
This is why you wont see many Luthiers use acrylics.... well, not perfectionist pains in the arse ones like yours truly here

But. If it works for you go for it!
Not every one has access to spray gear, let alone the space to set up and leave things hanging for months whilst waiting on finishes to fully cure, so acrylics and urethanes do have their advantages.