Uh you can 100% Legally modify the AGPL. The issue is not on whether if you can or can not, rather companies will find every loophole possible in your bullshit to try to exploit it, which is why I am saying to be weary on making modifications.

What exactly these limits are become a bit more difficult. The US govt has a 500 page book on all of the ins and outs of copyright regulation if you were really dedicated. here

Copyright law is also a constantly changing field with court decisions changing the grounds quite often for example here.

Software licenses are legal documents, and not something to be taken lightly. My point is that for modifications and for drafting your own licenses it is very important to consult people are qualified on such things because I have no fucking clue what exact bounds there are. I am not a lawyer.

It is just a risky move imo.

P.S. I found something about the use of these kinds of licenses, but little about their legal viability I think.