been down this rabbit hole myself. the main things to look at are grain capacity (based on your household size and water hardness) Reddit·r/homeimprovementideas
The "Fleck" part of the system is the control valve (the "brain" on top). While the tanks may vary by brand, these are the three most common Fleck valves you’ll encounter: buy fleck water softener
Buying a Fleck water softener is a smart choice for DIYers and homeowners looking for a "workhorse" system with readily available parts. Fleck systems, manufactured by Pentair, are among the most popular and reliable control valves in the US. been down this rabbit hole myself