honestly, I have to check with other people. You get kind of a blindness for how big or small your type is, and these forums are great for getting another eye to check you. Another way to check legibility is that both webtoon and tapas have a preview of how it will look on a phone, so you can just...double check.
As for type, I get a font that's easy to read while still looking handwritten--and if I do handwrite because I want it to look extra nice, I still use those fonts as a base to work off of--kind of like tracing. Handwritten calligraphy, even done digitally, is pretty meticulous, and you need a lot of guides to make it legible and clear.
But honestly, and this is me talking just my opinion, a comic intended for the phone and a comic intended for PC are inherently different. You can read them on both, but that doesn't mean they were intended for both, and even if you try and make a comic that works for both--it's always going to lean towards PC or Phone just because the difference in font and layout are So Different. I think most seasoned comic readers understand if they see a traditional page comic, to open a browser or a tablet. Likewise, seasoned readers on a PC will forgive a vertical format comic for it's large font face.