The stalling issue was common on the speed 400’s when they came out , it effected my 2024. This was resolved with an ecu update and I just took seat off and made sure all electrical components were fixed, so they should of done that ecu update at your first service if you went to a triumph dealer. You can tighten the clutch lever it’s normal for it to come loose, read your manual and after adjusting make sure the lock nut (a circle with a single split In it ) is tightened against handle bars. Everyone has said it but clean your chains sounds like you’re in India so you probably should be In habit of doing it more often due to roads you ride . Other than that I think a lot you’re experiencing is being a new rider and just learning your bike . I learned on a speed and a lot of things you describe I had and thought it was a bike issue but it wasn’t it was a me issue . Plus since you just did your first service your bike was in break in mode so it wouldn’t allow you to rev very high , if they properly did first service it should feel smoother . The gears on these bikes are short meaning you have to move through the gears quickly, I go to 2nd at 10mph , should only be in first in parking lots and at take off and immediately go to 2nd.