The best short term solution as far as I can think of is to introduce ethanol blends.
Brazil has been doing this for 40+ years, Thailand for over a decade.
USA and Australia has done this for over 5 years. Even UK started selling E10.
SL needs to introduce E10 / E20 / E85 / B10 / B20.
This will bring the prices down and reduce the amount of USD going out of the country as Ethanol can be locally produced via sugarcane, corn, etc. (Ethanol makes cold-starts a bit harder during the winter, but that won't apply to SL due to 24*C ++ temps.
But now, please don't start with "Sri Lanka cannot produce enough ethanol BS"
PS. This will also help with the low RON issue. Ethanol has a higher octane rating than pure gasoline. (E85 can have over 110 Octane)
Ethanol blends also burn cleaner than pure gasoline/diesel. So it will also help with reducing pollution.