I never used No Root Firewall, I have only used NetGuard, so I don't know the real difference, but I'm sorta stubborn in that I just use what I'm used to using. If it ain't broke and all that.
If you already disabled Download Manager you're good. That prevents any updates from any app stores or Google crap that might still try sneaking stuff in.
I don't use ANY app store, I just download direct APKs and use a lot of really old apps since I prefer the 2009-12 UI design era, and all the app stores offer are modern stuff I hate. My phone these days is used for very little more than an MP3 player and SMS messaging platform.
If you want to stop it from popping up about updates being available, you can try turning the internet off to Software Update in your No Root Firewall, and go into special access and turn off screen overlay/draw over other apps permissions for Software Update, set all notifications to silent (since it's not going to let you turn that off which sucks), then go into notification categories, select the remaining ones that are on, then set them to 'minimize notifications' and 'hide on lock screen', and then set the 'change system settings' to 'off'. That should silence them entirely. At least, that stopped my Samsung from doing it.
I personally am quite fond of Android 2.3, "Gingerbread" myself. Unfortunately there ain't one made that ran it that will work today because the carriers were dicks, so I have to work with what I got, which is a theme and a few older apps to recreate it, which is never going to be fully complete, and I can't change the status bar to look right either. Sadly I have to work with what I got. Maybe one day I'll buy a microcell and at least be able to use my Thunderbolt again within the range of my house.
