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What cards have a high spend bonus is constantly changing, and you need to evaluate what it earns relative to what you need. It's not likely to be a "sustainable" policy, ie, you may occasionally get a high-spend bonus which makes using Plastiq for two or three months worthwhile (you should look into whether you can your mortgage ahead of time when you have a high spend bonus which covers more mortgage than you would normally pay in the spend time window), but it's not likely something you can do year-round.

And you need to think carefully about different mileage programs. To start with, be aware of expiration policies:

Miles/Points that Do and Don't Expire

and you have to know how to "top off" if the miles you earn from the signup bonus are not enough for the award(s) you want.

Thus it works best for mileage programs you already belong to now and have other means of earning into (dining programs, shopping portals, car rentals, hotel stays or hotel program transfers, etc, etc). It's much trickier to use a high spend bonus for a mileage program you don't yet belong to and which you're totally unfamiliar with and wouldn't know how to "top off" or avoid expiration (without high expense).

Finally, you need to learn about the anti-churning rules at each bank, because that may lead you to apply first at certain banks (even if that's not highest spend bonus at that moment) because if you wait you may never be able to apply at that bank after that.