Heyho and welcome, Sebastian!
As Lesley already said, I was a little overwhelmed with all the work you‘ve posted. 😅 I can see that you are truly motivated to learn how to draw. And dont worry about getting a tablet, I found that some skills are even better to practise traditionally. (Such as the skills you learn with drawabox)
From what I‘ve seen, I think you will benefit a lot from practising the basic forms like cubes, cylinders, etc. And also pen control with straight lines...
I know, most people dont want to hear that kind of advice, because these exercises tend to be quite tedious and not as exciting as e.g. figure drawing or character design.
If you choose to do these drills, that will give you the biggest bang for your buck (at least in my opinion), make sure to balance it out with stuff you are excited about (=personal work).
I hope that helps a little (though its unpopular advice😅). If you have any questions, feel free to ask. I am really looking forward to your future work.
P.S. if you choose to post a lot and often, I‘d advise you to ask for specific feedback on stuff you are not sure about, instead of just saying „feel free to give feedback“, people will be more likely to respond👍😁