Greets and welcome to the Forum!
I started out with OnG as a kid too, we have that in common.
MonoGod lists - as I see them with the current Army Book - are very nice alternatives to give some variation once you've gotten some experience/understanding of how to play the Mixed list of Daemons.
I wouldn't advice anyone to 'start' collecting Daemons with the mindset of doing a MonoGod army though. There are people who play monogod in the sense that they use All units but model
them to be "all Tzeentch" or "all Slaanesh" for example - but that's not what I am refering to here.
I am actually in the midst of "playing through" the MonoGod armies of Daemons right now. Here is a
BatRep depticting when I used Nurgle Daemons vs Dwarves. Next up for me will be MonoSlaanesh! So I don't know, but perhaps there'll be some tidbits in there to give you an idea about the difficulties of going Mono.
Basically, while the possibility of playing MonoGod of course "exists" in a general sense, I don't think anyone will argue our Army Book is actually designed
for MonoGod-play. It isn't really a competative
alternative. In 7th Ed MonoTzeentch certainly was competative - but that's history now.