I have to admit that I'm a little overwhelmed with the all the options for care.
That's a pretty typical problem on here. We are each unique with different goals, climates, set-ups, flock make-ups, management techniques, and so many other things that there is not one way that is right for everyone or wrong for everyone. Even in the same circumstances, many different things work. It's not as simple as right or wrong. What is best for you might not be best for me, but either might work for both of us.
What is the best annual care routine for chickens?
My philosophy for overall health is to strengthen their immune system and rely on that as much as I can. My broody hens raise their chicks with the flock. I feed my brooder-raised chicks dirt from the run starting around day 2 or 3. That way they get any probiotics the adults wish to share, get grit in their systems, and start working on flock immunities immediately. I keep the brooder and their general environment as dry as I reasonably can, I consider wet unhealthy, but I do not try to raise them or keep them in a sterile environment.
I do not obsess over cleanliness. I don't like stepping in chicken poop and certainly don't let it build up to the point that it stays wet. I can tell by the smell when I've let it go too long. My main effort is scraping my droppings boards under the roosts to get that pure poop in my compost pile. One thing I think helps here is that I have a lot of room outside and weather where they can be outside all day every day. In winter you will not have that luxury. By them being outside all day every day in a large area the poop gets spread out. It doesn't build up to the point where it becomes a problem. There are two issues with that. If it builds up too thick it can stink but also the smaller area they poop in the more it builds up. Anything that lives in it or uses it as part of their life cycle can build up to a problem. I'm thinking of worms and Coccidiosis specifically but there can be other issues.
I consider clean water important. I don't put anything in the water, chemicals or supplements, some people do. I do want it clean. I don't do anything to their feed or give them supplements. Mine get regular chicken feed, forage for a fair amount of what they eat during the season, get kitchen scraps, and waste and excess from the garden. If I trap a mouse they get to eat it. They get a varied diet but nothing is in there specifically as a supplement.
As far as mites, lice, or worms, I do not do anything preventative. I only treat if I see a problem. I never have seen any. I check them for lice and mites regularly. When I butcher a chicken, which I do regularly, I look in the intestines to see if I can spot a worm. I never have. I give mine access to the ground so they can manage their own dust baths. I have never tried to create one for them and do not use DE as a preventative.
You will find plenty of people on here that have different opinions on practically any of these topics. They truly believe what they do is best for their chickens. I'm not trying to argue with them, for their flocks it could be. If my experiences were different I'd probably do some things differently too.
Good luck and welcome to the adventure. It can be a good journey.