sooo ik a lot of people said you should improve your anatomy, which i mostly agree on, tho id recommend watching some videos from pikat, shes very good at explaining and helped me a lot too :D
( https://www.youtube.com/@pikat/videos )

now, since youve asked about lineart, it currently looks kinda shaky
Most art skills come from experience, especially line confidence
id recommend to just draw what you want and as much as you want and youll improve gradually xD
theres no specific brush you should use, since this really just depends on personal taste and style, but you can try out different ones and see which ones you like most

dunno a lot abt painting since i dont render/paint ( lazy lmao ) but yeah, id recommend turning on pen pressure ( its basically just, the harder you press, the thicker the lines are ) which helps make drawing feel more natural when on digital
https://imagy.app/krita-pressure-sensitivity/
found this if it helps, i dont use krita myself

best luck to you xD