The basic plank is (typically) a rather easy exercise. This is mostly due to it being an isolation hold as well as being a well supported body position. If you can hold a plank for longer than a minute, then you should progress towards more challenging plank variations.
You have two options which build off of one another for plank variations. First you have positional variations. This includes side planks, bird dogs, one legged planks, one armed planks, hanging planks (regular plank while suspending the upper body by holding onto something overhead), and more. The next kind of plank variation is the dynamic plank. This includes cross-knee planks (and the same side variation), side plank oblique crunch, marching planks, mountain climbers, and more. The ab wheel rollout is arguably a plank variation too. There are many many plank variations out there. YouTube would be a good place to find these demonstrated.
One other thing of note here is your extra long rest time. The core muscles recover quickly, they really don’t need more than a minute between sets. Even 30 seconds is plenty, and if you consecutively do sufficiently different variations then no rest time between them is necessary except for when you reach muscular failure.