one method could be -
Create a new catalog pointing to all your photos, if you have one main parent folder, just import it. This will be the catalog of all your photos.
You can then copy the catalog and any associated files + your parent folder (with all the pictures) and move it into an external hard drive. Next time you open LR you can redirect to which catalog you would like to open + you can show it where the parent folder is (by clicking on the question mark on the parent folder in library module and directing it to the folder in your drive). Alternatively using LR, you can move the files to the external drive, so catalog already knows where they are. Then, move the catalog.
1-2 works best if any edits you made to your pictures in LR, are already pushed as metadata to individual files. You can do that by opening the catalog you did edits in, select all, save metadata to files. This will only create xmp files for your RAW files and add text to your jpg files. Then you can simply "read metadata in your new catalog.
You'll need to be careful about metadata compatibility between LR versions. Between LR versions, best to import photos from one catalog into another by using 'import photos from another catalog' function. It will upgrade and import correctly.
I'd definitely try it with a small test batch of photos, and backing up your originals or edits before changing directories and merging catalogs.
Another thought related to your situation, do you know if LR metadata can be read by your new software? If not the best way to push those edits might be to go into LR for individual catalogs you have, export all your pictures as "original" format (since you mentioned you want to continue to edit them, you may not want to go from RAW to jpg just yet) into folders and have these edited exported versions in addition to ones that are not edited. You can always add these to the catalog as well. That way your master catalog (Say you decide to open LR again) would have original images (with metadata side car files), and edited images.
There are lots of ways to automate individual steps as well.