>anyone buy kevlar sleeves?
No, because you can buy Slash Resistant Pullovers that are much better to wear. I have worn one of those together with a SK1 vest during my time as a professional Rapefugee-Herder in 2016 in Germany at a Rapefugee-Camp. They are useful against the Shivs and Kitchenknifes these type of people like to use, they protect your arms from being scratched up and are comfortable to wear. However they are only useful if you actually have to get close and personal with people.
>I've also read that material in bulletproof vests break down after two years, is that how often you need to buy them?
If they break down after two years that is a shit vest, usually the bulletproof vest used by the Police here in Germany hold 5-10 years. You can get a working vest already for 250€, by 10 years that is 25 bucks a year. If you buy a good bulletproof vest for around 800+ Euro, that is still just 80 bucks a year. Its pretty worth to spend 80+ bucks a year to have a chance of not dying a miserable death.
>The more stuff you wear, the slower you are in every way and the hotter it is.
A pullover will not slow you down and if its warm you will sweat in your vest regardless of what you wear. I once wore just a thin t-shirt and wide airy cargo pants and I was still sweating in my vest like crazy during summer.
>I have considered buying some chainmail armor to protect my torso and arms from knife attacks. Does anyone have experience with them and where to get one?
There are some companies in Europe that sell modern chainmail armor against slash and stab attack for security guards. One of them is Ziegler. https://ziegler-metallgewebe.com/en/industries/safety/security-shirt/
another is https://www.bsst.de/de/koerperschutz-shop/p9_secu.html
Expensive for just a Anti-Stab vest, but nice to wear compared to other Anti-Stab vest made of Nylon, Plastic and Kevlar.