Firstly, it's unlikely that a current compatible iPadOS is going th be the cause of your problem. The most likely culprit is lack of free storage space but there are other possibilities.
Secondly downgrading your iPadOS is a complex and risky Business. Just as an example here is an AI generated summary;
How to Downgrade iPadOS
"To downgrade the operating system of an iPad, you can follow these steps:
- Backup Your Data: Before proceeding, ensure you have a backup of your iPad using iCloud or iTunes. This is crucial as downgrading will erase all data on your device.
- Download the Correct IPSW File: You need to download the appropriate IPSW file for your iPad model from a reliable source. Make sure the IPSW file matches your iPad model and is signed by Apple.
- Connect Your iPad to Your Computer: Use a USB cable to connect your iPad to your computer. Ensure the connection is stable and your computer recognizes the iPad.
- Enter DFU Mode: Press and hold the power and home buttons for about 10 seconds, then release the power button but keep holding the home button until iTunes/Finder detects your iPad in recovery mode.
- Restore Your iPad: In iTunes or Finder, hold down the Shift key (Windows) or Option key (Mac) and click on the "Restore iPad" button. Select the IPSW file you downloaded and initiate the restore process.
- Restore from Backup: Once the downgrade is complete, you can restore your iPad from the backup you made earlier.
Note that you can only downgrade to versions of the iPad OS that Apple is still signing. Additionally, downgrading will erase all data on your iPad, so the backup step is essential.
For specific instructions on how to downgrade from iPadOS 18 to 17, you can refer to guides like the one provided by UltFone, which offers detailed steps including using profiles or third-party tools like Tenorshare ReiBoot for a simpler process."
As you can see, not a simple process. The most difficult part of which is obtaining a signed IPSW version of the older iPadOS. Failure to do this could end up "bricking" your device.
I suggest we look at other possible causes first.