@fm4nc-8466,Thanks for posting in Q&A.
From your description, I know you want to delete all person devices from Intune.
Based on my research, the device bulk delete action only support up to 100 devices once, due to you own few thousand devices, you can try to repeat the action a few times or you can create a PowerShell script to bulk delete the personal devices, here are some links with useful information you can refer.
https://techcommunity.microsoft.com/t5/microsoft-intune/device-removal/m-p/470419
https://github.com/microsoftgraph/powershell-intune-samples/issues/34
Non-official, just for reference.
Hope it will help.
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