They are willing to grind, cold call, door knock, and other prospecting more than any agent I have ever met.

That's my top priority for wish list items and no brick and mortar bank can help with that, lol.

That being said, I want to waste no time in figuring out what the next best move would be, especially as someone who's interested in the buy-and-hold strategy and in managing my own properties (at least in the beginning). 

Some say it’s possible to implement this strategy in Germany, while others claim you can't, due to regulations preventing the sale of mortgaged property before 10 years.There's a lot of confusion and misinformation, and no one seems to have solid information.This strategy is well-documented in the USA, with numerous videos on YouTube.

I'm contemplating if it would be better off in the future to have all long term units or if there's no cost difference for types of rentals. 

NO kidding way over thought and over kill.. a landlord with proper insurance and takes care of their property ( not a slum lord) has extremely low liability issues.. disputes with tenants is generally over deposits and when you go to evict them..

Multiple improvements done on this duplex in 2021, including new roof, gutters, flooring, drywall, electrical, appliances, and heating/cooling unit for the second floor.

And you are totally right, bookkeeping is a headache no matter what unless you're just really good at it, but necessary for your business. 

How come no one has created a standard form for hard money lending where the terms are easily comparable across them?

Since lenders can only lend up to 50% of whatever your gross income is assuming no debt.