You do realize you can release on both, and people that bought on Itch can be given a complementary Steam key?
https://itch.io/games/steam-key
Do you expect your bought game on itch to be supported long term and include all future additions ?
Uhhhhhm. Yes? Why is this a question. If you release a digital retail version of a game on a platform, it is expected to have the updates for said retail version included that would be included on every other platform as well.
If you release an early access version here, just make to sure to be specific what people will get. As long as you have not "released", it does not matter much. After release, Steam will have terms that will make you have same minimum price on all platforms. And if you plan to release on Steam and advertise as such, be specific, if a future Steam key will be included or not.
If you offer pay what you want, any payment will make a buyer eligible to download the project, even after a price change to minimum price. Which might be intended by the publisher, to reward the early supporters with basically a discount.
How do you feel about paid expansions to a cheaper base game
Depends. A lot. Mostly it depends upon the base game being a finished product or if it is basically unplayable without the dlcs and could be considered a bait.
People that like a game usually are inclined to try out or buy a sequel. An expansion is a kind of sequel. A loot box is not.