The solution is not complicated.
The problems seems because of bad quality engine oil & fuel. As you mention you are changing it after 2.5K, it sounds strange. Because now the good quality mineral oils of latest specs are even good to run upto 4~5K.
Use the engine oil as "ZIC A+ 5w-20 which is actually recommended by OEM.
Use good quality air & oil filter.
Fill 1/3 or 1/4 hi octane in the fuel tank always.
Change the plugs if these are done.
Use the spray to clean the throttle.
Your car rings seems fine.
Do above and share if the condition is improved.