Not sure if it's the "right way" to rule it or not, but the way I would do it? I would borrow an old rule from 3rd Edition that says if a number is doubled twice, it is considered a triple instead of a quadruple.
So Eldritch Spear makes the range 300 ft.
Then, Distant Spell doubles that, to 600 ft.
Then, Spell Sniper doubles it again. But, a double-double is a triple, so it's tripled to 900 ft.
Not that there's anything wrong with having 1200' range on a cantrip. (Within reason, of course. You still need to see something to target it...and some things might not be visible from almost a quarter-mile away because of their size, or because of rain, foliage, terrain, etc.) Past a certain point, the DM will have to make a judgment call on whether or not a target is valid at that range.