You’re right. To build strength you want to keep reps low and add resistance. You need to get creative. We use a weight vest for our stronger athletes for several of the exercises: 3 or 2 Point, Side plank. Wear boots for the leg raise and L sit.
As you get stronger you’ll discover that some of these Core exercises can serve as warm ups and you can focus on the tough ones for you and do more sets of them. Don’t be afraid to dream up others that may be more specific to your goals. Youtube is a great source for ideas.