With the MMM Mod and unique Tav, 3 (if you still use the alpha channel 4) tattoos should be possible at the same time, which can be individually colored, I mean also in metallic colors. Textures in BG3 work over the RGB + A channels and each can be assigned separate information. I use my own skin textures but I think some mods say "MMM compatible". This also allows different tattoos on different characters and I also think that it doesn't change anything in multiplayer. But yes, these tattoos have to be on one template, but then they could also be switched on/off and whether you like the tattoos is the next question.
I know about this, I did it but, from your own description, we can see that the way to install tattoos at the moment, via mods, is quite tedious.
And there are also other inconveniences:
as far as I know, these mods do not allow you to add tattoos, but they replace those already present in the game, which means that they also replace those already on some NPCs.
I am also currently using mods to have body tattoos, but (another problem), often these tattoos only work with the addition of the Unique TAV mod, but not all face mods are compatible with UTAV.
So, in short, it's a bit of a mess!
For this reason, through this thread, I would like to ask Larian if they could implement a section dedicated to body tattoos, so as to make it easier for everyone (so even for those who play on consoles), to have this possibility.
I know it's a little late to make such a request, but I'll try anyway.