(tl;dr: it´s probably not Foltik´s pack´s fault for no tank recipe, it´s fubar^2 from my side and I´m out of ideas. I´m still open for advices tho^^)
Ok, little update on the topic. Tried today to trun on/off some mods to see if there´s any chance for the recipe to appear or if there simply is ANY chance of salvation. And what I found is... beyond my comprehension.
First things first: Foltik´s pack is not the cause of trouble. Probably. I can tell you that much. Turned it completely off and still nothing changed. Was at least something bcs I could rule that one out now. But then, after some expreimentation, I discovered things that my (admittedly not very expericend) mind could not explain. Warning, we´re entering heavy fubar-territory, continue at own risk.
So I found out that the recipe for the small storage inline tank apparently is contained within the research "Electrolysis 1", which by my guess looks like it´s from Angel´s mods (whichever excactly it is, don´t think it matters to much at this point). And this research never appears ingame, it´s basically non-existant, which also would explain why there is no crafting recipe for the tank.
So did you find that one out, you may ask?
Well... I had this older savegame, where I never did anything, it´s sole intent was to load it when I modded sth in Factorio to check if it´s running and/or if there´s anything happening that shouldn´t. And if I didn´t spotted anything, I left it, being as it was. And this very savegame has said research and, behold, a working crafting recipe for the tank!
Now the thing is: I´ve tried setting up an entirely new world (with the same mods I use in all the other modded savegames), loaded it, but the research also wasn´t avaliable there. My mod-tryout-world is literally the only savegame I have where the whole tank-dilemma-thing isn´t a problem. So idk if the problem is just the game not being able to find the research recipe (despite not havin any error messages whatsoever), sth else or if my Factorio has simply given up on life. Might aswell be the latter.
idk if this helps in any way, shape or form or if I needed to live out my frustration and my need for communication. I figured since we already discussed it here I might add my findings aswell. And comfort Foltik that it´s most probably not his/his modpack´s fault.
If anyone is still bothered with this topic and bored enough to try to help me... hit me, I´m down for it :D