Tier2 changing employer

[Ysa]

Hi everyone
I m currently on tier2 (from Jan 2018 to Jan 2021). I left my job with my current employer and have an offer from another. So I will be applying for a new visa now
My questions:
1- should I apply under tier2 extension category?
2- I already spent 1.5 years on tier 2. So if I apply for less than 3 years visa, I will need to extend again to complete 5 years and be able to apply for IRL. My new employer is flexible and is willing to issue the Cos as per my request. If I asked for the more than 3 years option, how many years should be stated in the Cos?
3- if my cos stated 5 years for example, would the home office issue the visa for 4.5 years only according to the 6 years cap? Or this would be considered a mistake and the cos could be invalid and the application could be rejected?
Thank you in advance, appreciate your support