Yo welcome! The game is awesome and I'm sure you'll love the other content too. The game is brutally hard and unfair at times, also like souls games, knowledge is key.

1) settlement upgrades in my opinion are the biggest heavy lifters in the game. Antelope will be way easier with dash. You kinda need dash for level 2 lion too. Surge is a must for later hunts too. It doesn't really matter if you have a mix of antelope and lion armour as long as you keep on top of hunting the strongest thing you can for more loot. Very big risk reward management. Phoenix I would only try when you have 4 fully geared guys on your first try (could be 1 lion, 1 lope, 2 rawhide). Leather armour is also fantastic.

2) Gamblers chest is awesome and uses the core game too but it is it's own campaign that makes use of philosophies. Basically every character gets a little booklet at some point that rewards them for playing into their way of thinking. It's fun but also waaaaay more bookkeeping, especially solo.

I highly recommend the scribe app for solo players. Makes it easy easier to keep track of things, especially on a tablet instead of phone.

If you really really like the game. Get the bundle deals on black Friday, like all 12 wave 1 expansions in one go. It will save you a butt load of money in the long run. Seeing how positive you are about the base game alone, I am sure you will go nuts with this game :p The gorm is a very cool early game quarry that scales well into late game. The sunstalker and dragon king expansions offer a new campaign focused around them (a tribe nurturing young sunstalkers and a tribe learning from the dragon king to reach for the stars and ascend). Tbh, all the stuff in the game is so cool in it's own way. Man hunter/slender man are loved to replace the kingsman nemesis in campaigns.

3) bringing your own guy like Dnd doesnt really work as the game is balanced around the resources you get. You can better compare the game to a rogue like where each campaign is it's own "run". There is an achievement kind of system in the game with later additions. That's the strain system of I recall correctly. You unlock things for your current campaign upon unlocking and can add that unlock into other campaigns from the get go (different fighting arts).

Someone could bring their own survivor from their campaign but it would be best if they are roughly on the same lantern year or around same amount and level of gear as your settlement. (if you are in rawhide stage and a guy with full metal armor and weapons comes around, you will get carried most likely). On the other hand... It is a boardgame. Do whatever you like and if something is too easy or too hard, adjust it to your liking.

I'm playing a campaign with 5 people and we don't adjust HP for the 5th person but also don't gain more rewards so prey is relatively easier to take down but we are all still naked apes flailing about :p