RE: hmm should I level up VENARI MARKSRAT..?

It depends what league your playing at tbh fam! Ill give my opinion on each of them in each different rank!:)
Bronze: Doesn't make sense to level up because max level at bronze is 1.
Silver: Might be worth it to get to level 2 just for an extra health point.
Gold: Consider your playstyle. At this league he might better off at level 1 to make it easier to get the myriad to trigger.
Diamond+: Would definitely have this card maxed out as it offers a lot for very little cost!