is that the only service done to the bike?

wow, this is where the problem starts
the bike is neglected
google also can find this bike has oil filter

search a bit lah, regular maintenance stuff... oil, air filter, spark plugs, head service etc

Oh, it's a brand new bike? should go to SYM service center, right?

problem I found:
1. when full throttle, it is slower than say 80% throttle.
- it's a standard bike, not tuned to utilize max throttle

2. the engine is rough and vibrates slightly
- no idea, send to SYM

3. the meter glass/plastic cover vibrates quite noisy ketak ketak ketak sound when driving
- send to SYM, under warranty?

4. the gear is not smooth i.e. when I wanna enter 1st gear, sometimes it wont click until I release clutch and press again. it is better if I don't use the adjustable clutch to shorten it but it still happens occasionally.
- maybe the new oil is not suitable for the bike

tbh I can't help much
I googled VF3i oil filter, and can find it. Under a metal cover, paper element looks like.

some of the items are due by time, not only mileage - plugs and tyres are some of them

clutch can be mechanical or unsuitable oil, if you feel the problem arose with new oil, then change it. I've encountered many cases of poor match between oil and clutching, so each of my bikes use different EO

good luck

https://www.tiktok.com/@jmg_racing/video/7233600766676634881

another one inside oil drain plug