Hello Mc.bob. I would certainly mount antennas as well, greater range = faster WiFi. TX/RX refer to the antenna connections, if I'm not mistaken. There are 2 connections. One for 2.4Ghz and the other 5Ghz.
Your old card and the new one have the same standard 802.11ac. You're saying your old card is too slow. 433 Mbps is actually fine for normal use. Is the reception bad? Or do you have old devices on the WiFi network? Then of course the new card wouldn't do much good. If it's just the reception, does the old one already have antennas? Otherwise, you can just connect one of those and see if it gets better. And again as a hint: slow WiFi devices slow down all others in the same network.

Thank you for your explanations. I was hoping for 2 advantages with a faster card: 1. faster data transfers to the home NAS and 2. better utilisation of my gigabit internet connection. On my router I see that various devices are connected to the router on the 5Ghz Wifi network at speeds of 150 - 300. Is it therefore wrong to assume that a faster card could also connect faster to the router?

Compared to the Intel 9461 (1x1 without diversity), the built-in 9462 (1x1 with diversity) has two connections for antenna diversity. I therefore assume that two antennas are available. So nothing should stand in the way of an upgrade with the 9560 (2x2). Just install the latest Intel Wi-Fi and Bluetooth drivers.
My Asus Mini Barebone had a 9461 in it, but luckily the second antenna was built in, so I could replace it with the 9560 without any problems.