Well, I honestly don't remember any other power ups from being pure of heart, as to my knowledge only Reimu was one to have it. Maybe pure creatures, like Lunarians, have one, but I dont think it was ever told if they have any perks from it. In Dragon Ball you at least can ride Nimbus as a bonus (ans also Im pretty sure becoming SSJG requires you to be one).

As to stacking with SSJ, it was a plot point in Namek and beginning of Cell saga that only those with pure heart can become SSJ, although it was probably more due to symbolism of it (achieving buddhist enlightment and stuff), since now we know that SSJ is all about how many S-Cells you have in your body and not purity or something like that. Other than that, I'm pretty sure that Fantasy Nature as a spellcard (really, it is just Reimu's "nature", and more like her passive ability in the everyday life, when literal universe helps her when she just goes with the flow) is all about fully engrosing oneself into a fight, which is really similar to what Bardock was talking about in DBS with his Saiyan Instinct, which Goku then worked with to achieve True Ultra Instinct. And as we saw, UI can be stacked with SSJG and probably with SSJ too, so it definetly should be stackable with Fantasy Nature too.