Are multiple shallow cycles harmful for batteries?

Exactly! Solar of any amount is just a way to extend your time between recharges. You can play with how much solar, battery, and daily AH usage to adjust that time between recharges, but no matter what, you need to get the batts to 100% once a month. So the longest time you can shoot for as an extension, is a month between recharges to 100%.

How you get to 100% is just tactics. Shore power, gen plus solar, monster solar, smaller solar but split bank trick, whatever.