Well, I wouldn’t describe a Lamy 2000 - or any pen in its price-range - as a ‘starter pen’ 😁
But of course its cost may be trivial for you and, if it is, although I confess that I’m envious of your good fortune, I hope that you’re enjoying it, and sensibly too 🙂
In terms of choosing between the ‘F’ and ‘M’ nibs:
The 2000 is a fairly ‘wet’ writer, and my handwriting is small, so I bought mine with a ‘F’ on it, and I love it.
That said, I have read comments from other people on here who have found that the ‘F’ nib has too-narrow a ‘sweet spot’ (i.e. tolerance for variation in the ‘roll’ angle of nib-to-page) for them to be able to use it reliably.
Other people have found the ‘F’ to be too-narrow for them, and that the ‘M’ is perfect for them.
Nobody else can say which nib will suit you better. The only person who can decide between the two is you.
As you are apparently near a store that sells them, I urge you to go back to it, and ask them to let you write with both pens (dip them into a bottle of washable blue ink). While sitting down.
That way, you can find out which one you prefer.
Whilst other folks’ experiences can be useful to warn you about potential ‘problems’ that you may encounter with a particular nib, or a particular pen, your own experience/opinion after using it is the only one that matters.
Nobody else’s personal preference/opinion is in the least bit relevant to your choice.
Most Importantly: don’t rush!
There’s no hurry to buy a ‘good’ pen that will suit you, and which you will use for a long time.
Invest the little bit of time that is necessary to look-up pictures of the pens that are being recommended to you in this thread; decide which ones appeal to you, then; read/watch several reviews of each of those pens, then; try out in person as many of them as you can.
There’s no sense in rushing out and buying a pen that you may later find out that you don’t actually like that much, or that you don’t like as much as another model of pen.
Good luck