What else does Horizons change?
Lol well lets put it this way. It has its own wiki and you will need it for certain aspects:
https://fo4horizon.fandom.com/wiki/Horizon_Wiki
From my experience: it brings back a crude skill system in addition to the perk tree alongside specializations that you pick two times: one at lvl 15 and the other at lvl 30. Max level is 30 and by default the perks are limited to 30 (half of original). There is a perk called VANS that allows you to gain up to 30 additional perks (total 60 like in vanilla) with the downside that you lose the passive that comes from not picking it (5% increase of global damage up to 30% max at lvl 30).
The other thing is the health system (can only regen health with special items like bandages, trauma kits for limb damage, combat stims that are very rare and hard to make), the improved survival system (diseases, parasites, food and water requirements), the rescaled health system (enemy HP bloat gone, player HP increased significantly BUT its much harder to heal and much more expensive at the docs. Final result is that ping-pong health bouncy is gone) and finally rebalanced ammo (ammo rare, has separate craft benches, even when speccing in ballistics I regularly have to scavenged materials and return to craft ammo since my ammo drain is net negative).
Cant comment on the revamped settlements because I have not used it.
Seems very complicated but once you start using a lot of it is very intuitive. The big guidelines to take away is that
a)plan your general build from the start when you are choosing your SPECIAL distribution, including looking at potential specializations, which require perk preconditions. Some of them are very powerful at lvl 10 special BUT need lvl 20 min to access. You can respec but its expensive (1k caps). Caps are a lot rarer and for example even at lvl 20 (2/3 through leveling) I have accumulated 3k max caps between expenses like buying food or getting doc.
b)think through the various perk systems and look for potential synergies between them.
c) dont come in expecting fallout tier roleplaying because at the end of the day this is a survival scavenger shooter first and foremost.
I went in blind and after three levels of getting used to it activated the survival difficulty from very hard (adds food water, disease and parasited). The beginning in particular is brutal with no/low ammo and nearly zero health so all the meager caps you managed to scrape together goes straight to the docs big pot of gold.