Ameliaya wrote: …if I login into my personal icloud account on my work iphone, would those 8,000 photos be uploaded into my personal iCloud without deleting any of the existing photos in that personal account?
Yes-- the 8000 pictures will be copied to your personal account at iCloud.com. Also all the pictures there will be copied to your phone. Be sure you have enough room! If your phone or your iCloud storage get full, then things can go weird. If you're concerned, then you should turn on "Optimize Storage" on your phone. Then only smaller files will be transferred to the phone.
will they be uploaded chronologically?
You have no control over the order in which the pictures are transferred, but each picture will retain its "capture date" in its metadata. The order you see them in an album on your phone depends on how you choose to sort the pictures.
if yes, how do I do it step by step ?
(1) Sign in to your Apple Account, (2 )turn on iCloud Photos, (3) turn on Optimize Storage (4) at night, leave the phone plugged in, locked, with good internet, (5) wait a week or so.
See this:
Set up and use iCloud Photos - Apple Support