There is one last thing you can try/attempt, if you need to recoverolder versions of apps, for old or older, aging iOS devices.
If you own another much newer iOS device that can run iOS/iPadOS 13 and later, you can install the most recent version app/s onto that more recent iOS device.
Then go back to your older iPad, open the iOS App Store, go to the Purchases section of the store app, search for the app/s you wish to try and download to that older iPad and see if an older version of the app is allowed to be downloaded/installed to that older iPad.
These newer iOS/iPadOS devices must use your same Apple ID user account and password.
Failing that OR IF NO joy, then there is nothing more to be done.
It's 2021, now.
If you need/want an iPad with an up to date iPadOS with access to current, updated apps, then you really need to seriously consider a purchase of a new iPad model OR a much, MUCH “newer” and less expensive, refurbished or used iPad models, from better known, trusted sources, like other Apple product/device retailers/resellers, some locally or from online, on the Web/Internet, capable of running the latest iPadOS versions.
There are PLENTY of newer, older iPad models that can run the latest, current versions of iPadOS.
If you need OR ONLY able to purchase older, less expensive iPad models, look at older IPads from 2014 and later.
All of these older model iPads below CAN run the latest releases of iPadOS 15.
2014, 9,7 inch screen iPad Air 2 models
2015, 7.9 inch screen iPad Mini 4 models
2015-2017, 12.9 inch screen iPad Pro models. ( 1st gen Pencil support )
2016, 9.7 inch screen iPad Pro models ( 1st gen Pencil support )
2017, 10.5 inch screen iPad Pro models ( 1st gen Pencil support )
2017, 9.7 inch screen iPad 5th gen models
2018, 9.7 inch screen iPad 6th gen models ( 1st gen Pencil support )
2019, 10.2 inch screen iPad 7th gen models ( 1st gen Pencil support )
2020, 10.2 inch screen iPad 8th gen models. ( 1st gen Pencil support )
Get the largest internal data storage model you can afford as NO Apple iOS device has ANY expandable storage.
I am sorry that you had to learn all of this, but this does not change any of the facts regarding these older iOS devices.
Sorry & Best of Luck to You!