Thanks. Any others I should be looking at apart from the G84?
You still aren't telling us *why* you are asking the question.
Why is a keyboard tray not a better option?
How about a well that the board sits in to drop the height of keys from desktop?
These would give you a far wider range of choice than how you seem to be framing the problem.
Both would also seem to be better ergo solutions than a flat board but we have no idea why you think you want a flat mech board.
If you wanted to go crazy you could build a custom MX equipped layout straight into the desktop (cutting board style) if you have unlimited time, money and skills. If you wanted the key tops could could be flush with the desktop and you pushed the keys down from there. Sounds awful to me but maybe it is what you need/want.
If flat is the vital issue then you shouldn't be a switch snob and you should try the Apple, Logitech and Lenovo boards (not mechanical but ergo issues beat mechanical posing any day and ML is different enough from MX that the fact they are mech may not be as important as disliking how they feel). The key feel is not great on any of the suggested options but if flat is more important than key feel then...
Help us help you by telling us what your *real* requirements are.
This is like being at work trying to extract requirements from a client to work out what software they actually want.