Should I use premium service for FLR-FP

[shizo]

I was granted leave to remain on partner route and have a renewal due.
Unfortunately my partner and I split up in 2016 and we have a son who is British citizen.

As my renewal is now due, I now want to apply using FLR-FP (parent route based on my British child).

I am actively involved in my sons upbringing and have proof of involvement in his life such as expenses (upkeep allowance to his mum), holiday receipt, thousands of pictures, shopping receipts, letters from friends and his mum confirming my involvement.

However I have a few questions.

1. Is this a straightforward application, I am worried as I didn’t inform the home office last year that I split up with his mum. So is this any grounds for not being eligible to apply using premium service (same day in person)?

2. Are all the proof of involvement in my sons life I stated above adequate for my application.

3.I would appreciate any ideas of proof or anything to do to support my application.

Your response is much appreciated.