That is what you want to be doing , by topping up and maitaining a steady charge it will or should make the battery last longer , deep cycle batteries dont like being wiped out , they dont reckon that going below 50% charge is good , all batteries have a certain numder of charging cycles before they die , i bench charge with smartcharger every day , im lucky to get a year out of them so a solar maintained battery would seem to be the answer for me , plus i wont have to drag it in every night .
With regard to big versus small and warming up i really aint got a clue , but you seem to have hit a sweet spot with the bits you have .
Maybe i could run 2x50 watt instead of 1x 100 watt , toggle switch 1 or 2 depending on sunshine and battery level .