Thank you for your answer. Yes I realized that sadly the way SkillEffectInfoList.lub works is different, it's just an effect being placed on the screen, like some kind of overlay. If you rotate the camera it stays the same, unlike ground skills like Heavens Drive where there is an actual unit placed in the ground and if you rotate the camera you can see the back of that unit.
I thought of your approach with a fake npc but I realized it was way too much and that I should keep things simpler. For now I will stick with some 3rd job animation. Thanks anyway, I'm sure that will help other people with the same problem.
On a side note: newer clients (2019 and onwards) seem to use a new way of rendering ground skill effects, if you look inside the .exe file you will find that they have references to all these new str files for skills. And if you look in the grf you will find them divided by "ground" or "bottom" strs and normal str.
For example look at "new_earthdrive":
new_earthdrive_bottom.str: it's the one you show in the image above. It's placed "below" players.
new_earthdrive.str: it's some rocks shown on top of the _bottom.str
new_earthdrive_target: it's shown on the target hit by the skill.
I have no proof but from what I investigated it seems that before 2019 they hardcoded all ground effects because they needed to be actually placed on the ground and had to take into account a lot of things (just like rendering a character sprite), you can have people in front, behind, camera rotations, etc. And they put into .str the ones that are rendered as overlays, for example Magnus Exorcismus: the "angel" thing shown on top is a .str file, but the ground placement is hardcoded
But after 2019 it seems they are moving towards .str files for everything, even for ground placements.
So if you are a user looking to add a custom ground skill and you don't use 3rd/4th jobs then you can simply replace one of the 3rd/4th job skills str files and use clif_skill_poseffect to tell the client to render that skill instead. For example if you use Earth Drive then:
new_earthdrive_bottom.str: this is shown below players.
new_earthdrive.str: this is shown on top of players, like an overlay
new_earthdrive_target: this is shown on targets hit by the skill but I'm not sure if this would work with clif_skill_poseffect
Sadly I'm using pre 2019 clients. From what I tested newer clients had way worse performance (even with the vsync and sleep thing fixed), it sucks. Also a lot of changes that I would need to patch to make it work for a pre-renewal server.