Back when my kids were LL/CR age - we had a field like that which the middle school softball team used too. The roto-tiller is great to get started, but what really helped was periodic scraping with a heavy metal drag (using google try "DR Drag Harrow" - pic below). With teeth down it scrapes 1-2" off and keeps things soft. With teeth up it smooths things out. The other thing to keep close eye on is the build up at bases and not run the drag in the same direction every time. If/when budget allows getting some sort of dirt field conditioner (we used a product from Turface) added will help with being able to get games in when [heavy] rains come. The stuff wasn't cheap for our league, it sucked putting it down, but we got games in for years that prior to adding the product we'd have to cancel before.