As
@PoolStored suggested, the Circupool RJ30+ would work well.
There is the salt cell and a separate flow switch that needs to be installed in the return side plumbing.
The flow switch needs to be in a horizontal pipe - or in a vertical pipe where the water flows UP.
What is the distance from the 45 at the filter to the 45 in the header? That may be the better location to install both the cell and flow switch.
The flow switch needs about 12" of straight pipe before it. The length of the cell can count as part of that 12".
Consider putting the cell near the filter and the flow switch after the cell. Or remove the diagonal pipe and go vertical to fit it all in.
Remember, you will need to serviced the cell and flow switch from time to time, so plan on plumbing it in where you can easily access it.
If you are cutting pipe anyway and if the two gate valves are getting marginal, replace those now as well.
Also, consider putting some type of shade structure or shade cloth over all the equipment and plumbing to keep it out of the brutal AZ sun.