Thanks very much for your reply angelus9151! I had tried crt-pi already but while it looked ok-ish on some games it did not with others.
In the meantime I realised that I opened the box of Pandora with the question this thread is about since the possibilites are endless.
However I found two shaders that are more to my liking depending on what kind of look I want to achieve.
There is gtu-v050 for the look I was after, so kinda a CRT look but omitting typical CRT deficiencies but I also like crt-geom for a look that also emulates the curve of an older CRT.
I also found crt-mattias which delivers a look which introduces some deficiencies older CRTs had, so it looks a bit unsharp and the picture gets darker towards the screen edges. crt-mattias can set to show straight edges or curved ones. I must say crt-mattias does look very impressive in the games I tried but my AMD A8-7600 APU can't deliver 60 fps when using that shader except setting shader scale to 1x or 2x but then the picture isn't right.
I still need to fully try crt-royale and it's variants which are often recommended around the web, but my first impressions were not so good for my taste.
The above observations were all made playing 2D sprite based games, I've yet to try 3D stuff like N64 games for which another shaders is needed anyway. 3dfx is a shaders that's supposed to fit the original N64 look. I'll see.
I now still need to figure out how to make things stick, the biggest problem being that for example crt-mattias isn't in the Batocera release, I only found it by using the "Online Updater" menu option within RetroArch and updating GLSL Shaders via that which gave me a lot more shaders to choose from. However that update will be thrown out once rebooting Batocera. I know that Batocera is an embedded firmware type of OS, but for example LibreELEC is also and in that OS you can install addons etc. that are kept between reboots so I don't get why Batocera seemingly has all the RetroArch shaders and probably any other RetroArch components apart from some config-files in the read-only area.
Settings shaders per system is also horrible UI-wise, it should be adjustable from within Batocera's main menu which allows you to set many things but of the most important, the shaders, can only be set very rudimentary and with obscure (to the enduser) naming of the menu items.