For those thinking about jumping on the Charles Schwab Platinum before it's (potentially) too late, Lain did a great write-up in the past (and since I can't find it by searching, I'm just pasting it here ... I think this is all his words, and not any of my additions):
Amex Schwab Platinum by Lain:
There's no cost, other than a hard pull, to open a Schwab One brokerage account. And if you apply via a supportal link and can float $1,000 in the brokerage account for 45 days, you'll earn a $100 bonus. You may also want to open up a Schwab checking account while you're at it, since one of the benefits of Schwab's checking account is refunded ATM fees worldwide, which comes in handy when you travel internationally.
You don't need an Amex Schwab Platinum to open a Schwab One brokerage account, but you do need a Schwab One brokerage account to be eligible for a Schwab Platinum. And you need to keep a Schwab Platinum open as long as you want the ability to invest ("cash out") your MR to your Schwab One brokerage account at a 1 MR = 1.25 cent rate.
So to your question, there's no harm in opening up a Schwab One brokerage account early, long before you even intend to cash out your MR. The qualifying account will be there when you're ready. However, ideally, you'd open a Schwab One brokerage account via a supportal link and when you can float $1,000 for 45 days or more to earn at least a $100 bonus.
The other part to your question is when should you open a Schwab Platinum. Most wait until they've hit several of Amex's big SUB cards and built up a good stash of MR. This is because you need to keep your Schwab Platinum open for at least a year, but also for as long as you want the ability to invest ("cash out") your MR to your Schwab One brokerage account. If you want to keep this benefit for whatever MR you earn in the future, you'll need to keep your Schwab Plat open at a cost of $550 AF per year. Another reason is one of Amex's biggest SUBs is the personal Plat (whether the 100k MR Platicorn, or 70k/75k MR incognito offers, or even just the 60k MR public offer). Of course, you'll want to bank this SUB, but that means you'll also have a Plat for a year or two (if you accept a retention offer). The Schwab Platinum has basically all of the same benefits of the regular Plat (but with the ability to cash out your MR), so it's not really ideal to have the personal Plat and the Schwab Platinum concurrently. It's also not ideal to get the Schwab Platinum first then the regular Plat, because if you keep the Schwab Plat open to cash out you'll be holding the Schwab Plat concurrently with the regular Plat (and paying both AFs), or if you close the older Schwab Plat to avoid the duplication of benefits and another AF then you'll no longer have the ability to cash out any points you'll earn from the regular Plat and any other MR card. (Then there's a Morgan Stanley Platinum with it's own 60k MR bonus.) For these reasons, the Schwab Plat is usually a late in the game card, and for some the Platinum card they end up keeping (after churning the regular Platinum, Morgan Stanley Platinum, and the discontinued Ameriprise and Mercedes-Benz Platinums) for the usual Platinum benefits, but also because of its unique ability to cash out. (Of course others may take a hit it hard, hit it fast approach and get all the cards in a short period of time.) (One more thing: any personal Platinum — any personal Amex card — will add to your 5/24 count, so you need to decide how that fits into your strategy.)
There are always concerns whether this neat little benefit will continue. Amex and Schwab could decide to end their relationship, just as we've seen Amex discontinue the Ameriprise and Mercedes-Benz Platinums. Amex could decide to treat all the remaining varieties of personal Platinum cards with a SUB (e.g., the regular, Morgan Stanley, Charles Schwab) as the same product fo the purposes of its lifetime language restriction on bonuses, so getting a SUB on one variety makes you ineligible for a SUB on another flavor. Schwab could decide to stop the invest with rewards program or cut the redemption rate to 1 MR = 1 cent or less, since it's thought Schwab is footing the bill on the extra 0.25 cpp to get you to use their investment products.
Again, like I mentioned to you when you first started churning, Amex cards have serious AFs, but their rewards are lucrative and bonus categories are extremely useful, and having the right set of cards in your wallet can mean earning MRs at a much faster clip than even Chase URs. But because Amex cards typically have higher AFs than Chase's cards, you need to have an idea whether the SUB, credits, earning structure and benefits are worth it to help offset the fees, and a plan for what you're going to do with the MRs you earn. And as you know, Amex can be very sensitive to churning behavior, especially if you sign up for their cards and stop spending on them after you've gotten the bonus, then try to move on to the next one.
Investing your MR with Schwab, or cashing them out is one option if you have the right card. But MRs can also transfer over to Virgin Atlantic (frequently with transfer bonuses) and ANA, if you want to fly to Japan again, or British Airways or Air France/KLM if you wanted to fly to Europe, among many other transfer partners.
