I'd second or third the travel miles 101 course. I went through it a while ago and although I knew a lot already I learned a number of additional good things. I don't think they spammed me after completing the course (which takes only a week or three?), so the recurring email is a short term thing and quite well-behaved.
On expirations
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Most points and miles expire, although some don't. The way I manage it is to keep a list in a spreadsheet like this of program, current balance, and expiration date:
Delta Skymiles 13,242 4/21/2023
Citi TYP 12,141 12/31/2021
I then just highlight the rows that have high balances (for me anything over about 10K) and the next two or three upcoming expiration dates chronologically. I then have a tickler in my todo file to extend the expiration dates on the expiring miles or points. It's generally pretty easy to either use miles or points from the relevant programs (aka churn and burn) which usually also extends the expiration date, or to buy a magazine or accrue a point or two somehow to reset the date. There are plenty of web resources, just google "How do I extend the expiration date on my Citi TYP?" or whatever.
Some point programs don't expire as long as you keep the associated credit card open.
Last two questions
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I'm only single player, I think two player makes it easier to accomplish but harder to keep track of. Although in theory if you're organized well it should be reasonably possible.
Even in single player mode, I've been doing credit card bonuses and bank bonuses since 2007. You'd think it would slow down, but it really hasn't yet.
First, the credit card companies keep introducing new cards with new bonuses. So even though I've had the AMEX Charles Schwab Superb Fantastic card and gotten the bonus, I haven't yet received the AMEX Charles Schwab Preferred Amazing card with it's bonus (obvs made up names).
Second, some card offers are what's known as "churnable", meaning you can get the same bonus on the same card repeatedly. Often you have to cancel the existing card and reapply. Usually there are rules for how long you have to wait in between - with Chase it was 48 months on some cards, although I haven't kept up with the changes so their rules might have changed. Others are more lax in their restrictions, I think BofA is famous for having churnable cards.
Third, some card offers don't have the "once in a lifetime" restriction in the offer language for whatever reason. If you find those, you can use those application links and get the offer a second time even when you normally would not be able to. I've done a few of those and they generally work.
And fourth, if you have two players, you can each get each "once in a lifetime" offer, and you can tradeoff the waiting times, so you essentially have twice the opportunities that single player mode people do.
As for your last question in your OP, I use a collection of Excel spreadsheets. I have one that keeps track of my applications for a series of cards (I still apply for cards en masse, which was a trick from a long time ago that doesn't work but I haven't given up on it yet), as well as how much of the spending I've met, whether I've received the bonus, etc. I have one that keeps track of my reward programs (as mentioned above), and then I have one that keeps track of my current open cards and balances. I could probably come up with a simpler or more coherent approach, but this works for me.
