Welcome! Chase and AmEx are the most common cards we discuss here and that's usually where we recommend newbies start. Both have Marriott as a transfer partner. Chase also has United has a transfer partner. If you are wanting to accumulate points quickly, you probably will want to focus on Sign Up Bonuses (SUBs). Two important rules to remember as you get started in this hobby. 1. Chase has a 5/24 rule: you can only get 5 personal cards within a 24 month period. All brand personal cards are included in that count, not just Chase. You are currently 3/24 until your United card falls off in Feb. Many of us here have a sole proprietor side gig biz, so biz cards are in play as well. (Think ebay, FB Marketplace, consulting etc. Anything that earns you extra money.) That greatly expands the number of SUBs you can earn in that 24 months period. 2. Never close a card prior to the one year anniversary. Let the next annual fee post and then cancel it within 30 days. The AF will be refunded as long as you cancel within 30 days.
Older cards, in general, are good to keep because it helps with length of credit history. However, the CSP and the CF are both good churning cards. If you have other old cards, I would say cancel them so you can get them again with a SUB; or at least keep one of them open for now. Others here may have better advice on whether to keep those cards; I am definitely not an expert on that. You can only have one Sapphire card at a time, so you will have to cancel your CSP at some point if you want to get a SUB for a Sapphire card. Those cards can only be churned once every 48 months (most other cards are 24 months), and that's 24 mos from the date you earned the bonus (not when you opened the card). So keep all that in mind when deciding whether to keep or close your current CSP. Neither of the Sapphire cards have a good bonus now, so I wouldn't be in a rush to get one at this time.
If you don't mind a high fee card, you might want to try to pull a really high SUB offer for the AmEx Gold or Plat. Not sure what the highest available is right now. Hopefully, someone else will chime in with those specifics. There are lots of tips about how to "pull" AmEx offers if you do an internet search. If you get a SUB and can maximize the benefits, the cards often pay for themselves.
Good luck as you get started in this hobby!