Then help me, which card should I buy?
I'm assuming that by card you mean motherboard? (Sorry if that is incorrect.)
If you absolutely need more RAM, then any motherboard that has a decent VRM and the specific features you do want is fine. Stressing over which boards can do the highest memory speed won't matter because your limit for memory speed will be from the memory capacity.
If you want to stick with the Apex and just meant which RAM to get, the best balance of speed and capacity comes from 48GB (2x24GB) Hynix M-Die kits. I think GSkill makes a
CL26 6,000MT/s kit that is binned for very good CAS latency. It doesn't include a, 8,000MT/s EXPO profile, but the Apex should have a preset profile for that if I remember correctly. If you can't find that kit, some other memory brands may have similar kits.
If you want to learn more about RAM, the Apex is an excellent platform, but only if you pair it with the right memory.