Switching to H4 and H4 EAD from H1

I am working for company “R” on H1b and my visa expires on mid Aug 2021. They have not filed for my i140 or PERM so I
am not eligible to get further extensions. I am planning to switch to H4-EAD. My husband works on H1b with company
“A” and has i140. He is however planning to switch to company “B” and his interviews are in progress.

Should I plan to go on H4 and h4 EAD with company “A” processing for it. What if he changes his job to company “B”?
What impact will it have on my application? Do I have to withdraw it? or will it become invalid since my husband will change his employer?

If company “B” files his H1 transfer in premium processing, will I be able to file H4 premium? But I believe EAD will still
be processed normal timeline. how long does it take to receive EAD if H4 is approved in premium? The issue is we do not know how long it will take
for employer “B” to get him offer and start the visa processing so it could take upto 2 months and I may loose time. But I am also not sure if
I start the H4 process with his current employer and then he resigns what impact it may have on my application?