I have both the CSR and Platinum and plan to keep both at least for the foreseeable future. My wife also has her own CSR and is an AU on my Platinum.
We both got the CSR for the 100k UR signup bonuses, but have since found it to be an invaluable card. The 3x bonus categories for dining and travel is well suited for our lifestyle and make this card a very fast earner. Of course, this means paying two $450 annual fees but we have no problem using up the $300 travel credit on each of our cards, lowering the effective cost to $150 each. If we were to cancel one of the CSRs and put one of us on the other card as an AU, we'd be paying another $75 anyway, which practically lowers the cost to maintain an additional CSR at another $75. That's ok with us.
If you got a 100k MR bonus offer from Amex for the personal Platinum, go for it if only for the points. I believe that's the max offer for that card. Of course, see if you can go for this offer after you've finished with your Chase cards when you drop below 5/24 soon. (Although, I've read that new Amex cards can take a month or two to show up on your credit report and you can sneak in a Chase app or two before a new Amex gets reported. My new Amex from mid-February was finally reported this month.)
I wish I had gotten a 100k offer for my Platinum, but I've never been targeted despite trying, so I finally settled for a 75k incognito offer. I did it mainly to pad my MR points, which had gotten pretty low. The plan is to eventually transfer my MR points to Hawaiian Airlines miles when a 25% transfer bonus comes around again. Like
@calypso726 said, it takes some advance planning. If a transfer bonus doesn't happen before we have to book our flights for our Hawaii trip later this year, those points will still be there for the next time. Hawaiian is our preferred airlines to Hawaii and we try go every year, so the bonus is like two free roundtrip tickets. I also made use of the Platinum's airline credit by purchasing two $100 Hawaiian Airlines gift certificates, and I guess that's how I'll be spending the card's $200 airline credit every year.
I also got the Platinum for Centurion Lounge access because my wife has been flying increasingly for work. I'll keep the Platinum because we fly out of SFO so we have a Centurion Lounge here, which we prefer to the Air France/KLM lounge offered by Priority Pass. I don't think you have a Centurion Lounge at your home airport, so this probably won't be a deciding factor. It's a little sad to think I'm keeping the Platinum as a glorified lounge pass, but otherwise the benefits aren't too compelling for me. It's not a fast points earner at all. I don't find myself or my wife using the 5x bonus on flights and hotels; and the Platinum is 1x for everything else. Automatic Hilton and SPG Gold status is nice. I haven't used the Platinum's Concierge service, but if we didn't have a toddler who's too young and high energy to sit still, I'd definitely call Concierge for Hamilton tickets or French Laundry reservations. Uber... HA! I hate the company and their whole business model, but find myself having to use up the meager $15 Uber credit Amex throws at me every month to justify the new $550 annual fee. That's $200 a year! So I treat the family to delivery by Uber Eats once a month at a $15 discount and pay for the rest using my CSR, which codes as 3x travel.
And that kind of goes to the problem with the Platinum: I don't really have any reason to use it to pay for anything when I can put it on the CSR. For dining and travel, the CSR has the Platinum beat with 3x bonuses. Add in our Chase Freedom with rotating 5x categories. For shopping, the CSR offers price protection (Amex doesn't) in addition to product and return protection. So I find myself just putting a lot of recurring charges on my Platinum -- at least for services that accept Amex -- and my nice shiny, new metal Platinum card has barely been swiped.
Anyway, I agree with
@calypso726. Get the Platinum for the 100k bonus and if you don't find enough value in the card to justify the high annual fee, downgrade to an Everyday card to keep your MR points, or call up Amex around when your next annual fee posts to see if they won't give you some kind of retention offer.