I would rather avoid Tik-Tak as meet-up location, as well as similar bars/cafés. Loud music/loud crowd, little light, much smoke… These reasons are behind my suggestion to go to somewhere more restaurant-like.
I myself will refrain from proposing further places, as too many options won’t make it any easier.
In case that this is not an April Fool’s Day joke, I’ll try to come. Sunday seems good enough (though 12:00 is a bit early-birdy...)
As this is at lunchtime, I’d vote for something like Koko, Südtiroler Platz: The food is good, it offers more than one vegetarian dish, and it is not only in walking distance to just about all the inner++ city of Graz, but also reasonably close to a parking place should anybody arrive by car.