Im doing planks after set of leg raises to kinda finish my abs. Im just holding it as long as I can (or a bit less). Also when Im doin plank I place my hands closer together, feet wider (shoulder width or less) and slowly touching side of my legs with one arm (alternating them of course) and trying to not move/twist my body at all to challenge my core to stabilize my body.
Anyway you can build time up to few minutes, then try some harder excercises.
Once you can comfortably hold a plank for 2 minutes, I suggest start working on L sits. They are both isometrics but L sits will take you much farther and help prepare you more for levers. Also with planks you can work leg raises concurrently building toward full hanging leg raises. Once you can do those and L sits comfortably then you will probably be ready to start training on the more advanced levers.