Which RTK do you like?

I am preparing for a swarm flight this time.
It seems that sophisticated GPS is the most important.
I don’t have money, so I want to control 5 to 10 quadcopters

The distance between the drones is thought to be about 5m.

Which RTK do you think is the best for swarm?

Looking at all the RTK descriptions, everyone says that they are the most accurate and superior.
What should I believe?

The cheapest from them all seams to be Here 3. I do not know how good it is.