There is simply no way you are going to get real accuracy with single frequency (L1) GPS systems.
All survey grade systems use multiple frequencies as well as multiple constellations, and I would be very cautious of claims of great accuracy with single frequency as they have been proved time and time again to just not real.
Here is one discussion
and there are many more discussions if you do a search.
The biggest issue I have seen with single frequency RTK, and this is from discussions, not my own experience, is that it loses lock and takes a very long time to get lock, and then loses it in use.
We are using an AsteRx-m multi frequency which gives a solid lock, even at ground level, and an x/y accuracy of +/- 6mm.