Again, I'd like to thank everyone for your advice, which inspired me to carry on without fretting about arm movements etc.
I'm posting my monoline Spencerian after almost 3 months of semi-regular practice. Of course, with self-isolation the frequency and length of practice has increased.
These days I'm using a Lamy Safari, EF nib, J. Herbin Vert reseda ink. When I compare this sample to my January writing, I can see a lot of improvement, but also many mistakes

I find that the more you practice the more observant you become.
The only downside: I started practicing this hand with the goal of making it my handwriting, now I'm less convinced it's viable. It's really easy to stray away from slant without slant lines.