The town of Eatonville could be seen as a kind of heaven in multiple ways. As you suggest, the all black town is a kind of haven for African Americans where they can live and thrive is a place apart from the racism of the white world. However, there are limitations to that perception. Ultimately, Eatonville is no different from any other community that exists; we see this is the portrayals of gossip, jealousy, greed, sexism, etc. that are explored throughout the novel.
Another way we can see Eatonville as a kind of heaven is that it serves as a seat for Joe Starks who sees himself as a God-like figure. From his insistence of the incorporation of the town, to his rather rough dealings with the townspeople, to his oft-repeated catch-phrase, "I God," Joe perceives himself to be the ruler over everyone and everything in Eatonville. We see this idea most clearly when Joe makes his speech at the lighting of the first lamppost, which Joe himself purchased, in the all-black town:
"Folkses, de sun is goin' down. De Sun-maker brings it up in de mornin', and de Sun-maker sends it tuh bed at night. Us poort weak humans can't do nothin' tuh hurry it up nor to slow it down. All we can do, if we want any light after de settin' or befo' de risin, is tuh make some light ourselves. So dat's how come lamps was made. Dis evenin' we'se all assembled heah tug light uh lamp. Dis occasion is something for us all tuh remember tuh our dyin' day. De first street lamp in uh colored town. Lift yo' eyes and gaze on it. And when Ah touch de match tuh dat lampwick let de light penetrate inside of yuh, and let it shine, let it shine, let it shine."
The tonic that you suggest that exists in Eatonville, however, comes more from Janie's experiences in the town. Eatonville is where she has spend the vast majority of her adult life; it is where she finally found her voice on the store porch, where she finally stood up to Joe's mental and emotional abuse, and where she exerted herself as an independent woman. Despite the sadness at Janie's loss of Tea Cake, she returns to Eatonville a self-actualized woman, victorious in her quest to find herself. Her return to Eatonville is a kind of celebratory dance to both celebrate where she has come from and all that she has become.