First time visitor - looking for assistance with 2017 Octavia rs

Hi all, I hope this is okay to post here - I am looking for some help.

Just under a year ago we purchased a 2017 Octavia rs sedan. It has been great until recently. At start up, it often runs quite rough or dirty, and you can feel it shudder a bit as it warms up. Is this normal? We have only noticed it recently. We have recently had it serviced as well and that didn't ease the shuddering.

Further to the service - the car computer tells us "oil service due" despite us already having done it. The garage we took it to clearly didn't update the computer. I went to try this myself, but the user manual says "not recommended for variable service intervals" and doesn't provide instructions on how to do this. Is this really something a garage has to do?

Sorry if these are stupid questions. I'm not much of a petrol head myself so it's not really an area in confident in.