Comparing the 2021 fixes with your first 2016 build, filesync shows me that there are 795 files that are different. Somewhere in those files I guess there are fixes for the bugs that appear now with this 2021 build:
spyglass still not working
wrong icons in battle interface menu
incomaptible savegame everytime I save a game (that used to work)
when firing cannons, only certain cannons show smoke particle
some ship and character icons and desriptions missing when choosing ships/characters in Free Play game.
enemy ships surrender too easily (I don't know if this is a bug or made with purpose like that on original NH, but it sucks)
SHIP's LOG menu is not working when in F2 menu
EQUIP button is missing in Inventory menu (used to work in 2016 build)
storms have flat waves while in original NH they look pretty scary :)
If you could point me which files to look at, I could try to fix those. Thank you very much for all you are doing, I'm trying to make it as playable as possible, so you can post the new update here on itch.io
P.S. for storms I know that I need to fix the sea harmonics in Daystorm.c so I will fix that by myself, because it's same as with GoF mod.
Also, how to get rid of this ugly ===start_storyline.c===, it now seems to appear everywhere, in 2016 build it didn't...