Hi Anna,
Please make sure as Ralph said above that all your photos are safely on the computer and you can view them there without difficulties. Follow his instructions, please. Make sure the 3,500 hundred that are on your iPad are also in your computer as long as the other 30,000. Likely the 30,000 already include the 3,500.
With My Photo Stream ON, you are automatically uploading new photos to iCloud and sending them to all your iCloud devices when they are on Wi-Fi. This can be a real convenience or it can be a nuisance depending on your perspective.
You are using iCloud Drive and you should read the FAQ before you turn it off.
iCloud Drive FAQ - Apple Support
Photos are extra special and here is what I would do. Once you can see that they are ALL on your computer, use an external hard drive to back them up. Your 30-35 thousand photos may well take about 80-100 GBs or even more and these days you can buy a 1 TB external drive for about $60. Then before you do anything, make a backup copy of all your photos to this external drive and you will never lose them, even if the computer crashes. You will then have to decide if the advantages of the iCloud Drive are worth it to you. I do not use iCloud Drive nor Photo Stream so all my pictures are independent in each device. I have all of the items above set to OFF but only you can make that decision once your read what you might be losing.
If you decide to ditch iCloud Drive then make the backup copy above to be safe (the pictures should stay on your computer) and then delete all you want from the iPad.
Cheers....