Been a bit since I’ve messed with custom ROMs and rooting, but I believe it’s still possible to customize the look of things on ROMs, especially if you root it as well. Might be able to at least mitigate the stuff you don’t like.

Either way, it might be worth asking, do you care more about your OS being ugly or do you want a good messenger like Signal? Because with Signal’s focus on security you’re not getting it working on Android 4.x or earlier anytime soon.

Also, note as someone who works in cybersecurity and hacks things for a living, you might want to just bite the bullet and upgrade anyway. Old Android versions like that are SUPER easy to hack. Like so easy it’s used to teach people who are new to hacking the basics. Basically the equivalent of running Windows XP or 7 today.

That is quite possibly the stupidest argument against updating.