To the OP, something that hasn't been mentioned yet is that flat vs. clipless does not have to be an either/or decision. I ride with SPD equipped shoes on all my bikes, road included. However, on my MTB and soon on my gravel bike I use a pedal that is flat with pins on one side, SPD clipless on the other. You can decide at any moment which will work better for you right then. I find climbing to be easier and more comfortable "locked in", but sometimes starting, especially on a grade, is much easier with the flats.
Eric