I have a Schwab One International - I've never had the (US-domestic) Schwab One or checking accounts, but I do know a few people who had to switch when they moved. I don't believe you have a choice in the matter - most US brokers will completely shut down your account if you move, Schwab at least let you keep it with some restrictions. The only alternative is to lie to them about your place of residence.
AFAICT the Schwab One International is identical to Schwab One for investing, except that you can't buy Funds (you'll still have the full range of Schwab ETF's). You can blame US law for that: no serious broker will sell US funds to persons not resident in the US.
You'll still have a debit card with the same free withdrawals, and you can still get checks - and you can still deposit checks using the app. One significant difference is that you can no longer have a full checking account (aka no High Yield Checking account) - that also means that your cash isn't FDIC insured since it's not in a full checking account. But you can still use the account for ACH (e.g. for moving cash to other bank accounts, or wherever else you'd use ACH).
If you move back to the USA, I believe you can just convert back to the domestic version of Schwab One, I don't know of anyone who's actually done that though but that's what customer service has told people who converted in the other direction.
Regarding the 401k: I don't have any experience with the 401k, but I know of people who've opened IRA's or rollover IRA's. The online forms only support addresses in the USA, but people have been able to open their IRA's by submitting the pdf form (you can submit it via the Schwab website online messaging). I'd recommend trying that.
The other good option for investing in the USA is Interactive Brokers: they're not free, but only charge approx 1 USD per trade (also: there's a 10 USD monthly minimum commission if you invest less than 100k with them), and they offer by far the cheapest currency conversion I've seen. Opening an account with them tends to be quite fast (faster than Schwab), as long as you have the necessary proofs of address etc. But I don't know if they offer the 401k you're looking for.
FWIW if you're USA taxed (the solo 401k suggests you are), then you'll want to avoid investing in non-USA domiciled funds or ETF's due to the (protectionist) PFIC rules, so you pretty much don't have a choice in using a USA based brokerage.