Current control and more for lead acid battery charger

[okw]

Hello! I'm making my own lead acid charger (6V and 12V, max 4A). I know there exist plenty of example designs, but I wanna do it myself (for the learning, and the customisation).
Input will be from a clean 20V DC supply I have, and charge voltage will be regulated with a switch mode step-down.

I'll start with the most important question, how to adjust current? I've attached a schematic of two N-channel MOSFETs and a digital pot (drawn as a resistor for now). Will this work? And how to calculate resistor value based on how much current I want to pass through? Are there better ways than this? I could of course replace the pot with 2-3 resistors and transistors to have fixed charge levels (e.g. 0.2A / 1A and 4A), but it would be sweet to be able to adjust over the full range. I think maybe put PWM + fixed resistor could do the trick, instead of potmeter tied to 3.3v?

I see most lead acid charger schematics are based on linear regulators, but are there any drawbacks of using switch mode regulators (except complexity and cost)?
Anything I should take into consideration, avoid or pay attention to? I've designed a few DC-DC circuits before (but only as power supplies, not chargers).

It shall also monitor both input and charge voltage and current, as well as adjust charging (voltage and current). Adjustments shall be done from an ESP32.
I've used dedicated current monitors like INA219 in the past, but this time I would like to play around with opamps. I'll chose a dual opamp with input offset of 1-1.5mV (or better). That's Ok for rough current measurements? How do I choose the proper resistors? I have now (for charging opamp) 1k/13k which gives me 2.912V when 4A passes the 56mR shunt (if I'm not mistaken). Should I choose higher/lower resistors (less parasitics vs more noise resistant)?

I can adjust the voltage by connecting a digital potentiometer as feedback resistor on a very simple and cheap DC-DC buck. The only thing that would differ between 6.8V and 13.6V output is the inductor. And 95% of the times, I'll charge 12V batteries anyway, so I probably choose an inductor for this output(?).