So many seal cards but nothing left to catch

I've now got cygness, scythe, neon and meniss

What do I use my seal cards for now? I need some more rare monsters in caves to catch

If you have a breeding pair of those monsters, then, you might want to hold on to your seal cards for now. When you are preparing to get your legendary monsters, such as Gryphion and Hippogon, you would want to use pure S monsters and you need blessing to ensure that this will happen.
I went from 300 cards to 52 cards in 2 days just to get the monsters for blessing.
You will definitely need these so try and catch monsters now for sacrifices later. You can also capture those monsters to triple the EXP you get from the battles - a good way of levelling your monsters.

Good luck!