I've been sewing a long time - I'm tall and have had to make my own clothes for years.
It probably wouldn't be a difficult one to draft especially as it's a loose fitting garment so much more forgiving on measurements being precise.
Princess seams can be a bit tricky to sew over the bust but they just need practice and finding a technique thst works for you.
The tunic book I got is The Tunic Bible : https://www.amazon.de/dp/1617453560?ref=ppx_pop_mob_ap_share it's made me excited about sewing again!