Patience is important, messages might not start vanishing for a few hours but quota is freed up.
Hoping you have Exchange Online Plan 2 license so you really have the over 50 GB available.
Set-Mailbox firstname.lastname@example.org -ProhibitSendQuota 98GB -ProhibitSendReceiveQuota 99GB -IssueWarningQuota 97GB
Clean up things? Again, it may take a few hours to see the numbers start shrinking.
Change that hold to just 0 days then?
Set-Mailbox email@example.com -RetainDeletedItemsFor 0
Make sure there’s no litigation hold on anything?
Set-Mailbox firstname.lastname@example.org -RemoveDelayReleaseHoldApplied
The run a clean up?
Start-ManagedFolderAssistant -Identity email@example.com
Search-mailbox -Identity firstname.lastname@example.org -SearchDumpsterOnly -DeleteContent -Force
Start-ManagedFolderAssistant -Identity “email@example.com ” -HoldCleanup