iMessage routing with multiple devices - Forcing SMS delivery
Anyone that had an iPhone can now de-register that phone number from iMessage delivery at https://selfsolve.apple.com/deregister-imessage

In the past, you had to know to turn off iMessage before you removed the SIM from the device to accomplish the same thing, but clearly enough people needed this that it's a self-serve option now from Apple to manage your iMessage status.

Then you can choose to SMS them or iMessage them and know when the iMessage is really landing at a device that's capable of replying to it. It doesn't perfectly solve your case, but for many it will be an ideal way to get messages reliably from non-iPhone devices.