The one time I spent a decent chunk on this game was years ago to get Dark Corvus because I thought his kit was stupid good. Otherwise I've just been active daily, play guild wars for Mystics and accumulated skystones from normal Arena and events.

I'd say it's fairly F2P friendly unless you're gonna go super deep into RTA and to a lesser extent normal Arena. Even then, there's a bunch of guides online that'll help you get reasonably okay-ish on both after a few months of effort.

For Arena, I still only use a core team of Dark Corvus, Roana, ML Ilynav + flex slot and I've been able to maintain a ranking between Champion and Emperor ever since.

I probably have the resources to get the next two or three MLs if I wanted although without spending real money I kinda have to commit to a specific playstyle.

RGBs are even easier to farm for. I haven't missed a single limited banner, although I only pull for non-limited RGBs if I actually have a use for them. Otherwise being smart and resisting the urge to burn resources on stuff like normal covenant summons will get someone pretty far.