How will icloud & old photos library affect new one?

Had a recent death on in-law side and daughter was responsible for collecting photos. She had miserable time as they were scattered all over. As the 'unofficial' photo keeper/organizer for my side, I want to make it easier when..... I also read an article about some loosing their access to icloud along with all their pics, etc. So I began to look more closely at how photos has my 28000+ family photos arranged. Boy it's a mess. Lots were scanned and imported and the dates are all over the place. I think the only thing I can do is go back to the original HDD backup when I keep all pics in Lightroom and import them back to photos, but arranged more orderly. My first idea was to export all that I do not have already backed up to a HDD, then completely delete/empty my photos. Secondly I thought I could create a new blank library & import photos there. Once the new library is all setup, i could delete old library -- but -- what about those pics still in icloud? I want nothing from old library to infect my new one. Suggestions for what to do?