Reddit mentions of Modern Greek

I have a few (untested) suggestions!

I've heard good things about Modern Greek by Theodore Papaloizos who apparently wrote all of the Greek Sunday school books.

I have Greek Today which I used in a college class - it's super dense and extremely technical upfront. It's also designed to be used in a class setting so not sure how effective it would be for self study. I have a grasp of the language as a heritage speaker but this really goes all out with a lot of grammar upfront (which is hard for me to muster the motivation to get through).

Finally, I have Greek Now marked in my running list of Greek books but I can't remember what the deal was with this - I know I found out about it through this sub but search for "Greek Now" or "Ellinika Tora" and you may be able to get some better background.

I am actually using Assimil for French and I personally love that methodology! If you are able to power through the Greek version in French, I think that should be super helpful!

Edit: Agree with CaucusInferredBulk about the Routledge grammar book - I have the essential and it's very useful as a reference guide once you get to the mid to advanced level.