Incorrect. Firstly, the Tier 2 route ceased to exist several years ago, it is just the skilled worker route now. There is also no PBS dependant route at all. Secondly, your dependant's visa remain valid until expiry, regardless of whether you get ILR or not. There will be no such visa side effect at all.
The issue dates you provided suggest that you got visas close to each other, so yes she can get ILR with you. You have to wait and apply with her around November, which will be the earliest date of her ILR eligibility (23/12/2020 + 5 years - 28 days).
By that time, you have already qualified anyway. No need for an expensive extension for her.
Make sure to read this:
Applicants can benefit from the date of the ILR decision
Advice is given based on my personal research and experience only. Do NOT contact me via private message for immigration advice