ILR - PBS Dependent Extension Query on IHS

[aayushahuja]

I am currently on Skilled Worker Visa (5 yr) & will be completing 5 years in UK in Feb 2024; I will be applying for ILR in Jan 2024.
My spouse is on PBS Dependent Visa with her BRP expiring on the same date as me in Feb 2024. She has only completed 1 year in UK so cannot apply for ILR with me, and hence will be applying for her PBS Dependent Visa Extension.

I have a few queries -
1. Can I submit my spouse's application immediately after submitting mine, using the URN number of my application or do I wait for my ILR to get approved before submitting the application?
2. How much IHS do I have to pay - for how many years?
3. The IHS payment website asks for main applicant's visa expiry date? Since I will get ILR, what expiry should I put in?

Many Thanks!

[aayushahuja]

Related to (2), will my spouse get extension for 3 years, 4 years or 5 years?