Everyone is still speculating about this, but most have been leaning toward the no-cost option being a Gmail address (https://9to5google.com/2022/01/27/g-suite-legacy-transfer-gm...). The wording on the Google support page mentioning the no-cost option is still vague enough to have me doubting this as well.
If the no-cost option isn't Gmail, then there's the option of obtaining new Gmail accounts and transferring the data to them manually. Then the ImprovMX option should work.
I have many legacy Google Workspace domains setup for friends and family who are not tech savvy. They are all comfortable with the Gmail interface, so it will be hard to move many of them to a new interface.
I have looked at the companies mentioned and came to much the same conclusion. Currently I am contemplating ImprovMX (https://improvmx.com) which could provide an SMTP server and allow those domains to continue to use Google's no-cost option (https://support.google.com/a/answer/60217#nocost). If this option is anything like GMail it should be possible to setup the 'send mail as' account as the default to continue sending and receiving mail from the custom domain using ImprovMX's SMTP.
I have a good size community on KeyBase that uses it regularly. We tried to switch to other platforms like Signal, Matrix, Session, and others, but the one that everyone feels comfortable with is KB.
If the no-cost option isn't Gmail, then there's the option of obtaining new Gmail accounts and transferring the data to them manually. Then the ImprovMX option should work.