As we cannot do magic, there is no other easy way to solve the problem of multi-resolution, without special server software, etc.
However, you can already significantly reduce the number of images yourself, with the drawback that it may put a heavier load on the user's hardware. As newer phones are more capable, this may not be a problem.
If you change for example the level tile size to
1022px you will get just a quarter of tile images.

- Screenshot 2019-04-26 11.08.30.png (87.04 KiB) Viewed 600 times
If you always have the same size for your input images, you can also set manual levels to have the largest level as a multiple of the level tile size. This avoids smaller tiles on the borders. An example would be levels of 4088px, 2044px and 1022px if your equirectangular images is ~ 12000 to 16000px wide and your tile size is 1022px.

- Screenshot 2019-04-26 11.10.34.png (32.55 KiB) Viewed 600 times
Please remember to delete the output folder before you generate a new output with a different tile size, to avoid older orphan tiles in the output folder.