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 thebigbox@windrammer.net -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 thebigbox@windrammer.net -RetainDeletedItemsFor 0
Make sure there’s no litigation hold on anything?
Set-Mailbox thebigbox@windrammer.net -RemoveDelayReleaseHoldApplied
The run a clean up?
Start-ManagedFolderAssistant -Identity thebigbox@windrammer.net
Extra Cleanups
Search-mailbox -Identity thebigbox@windrammer.net -SearchDumpsterOnly -DeleteContent -Force
Start-ManagedFolderAssistant -Identity “thebigbox@windrammer.net ” -HoldCleanup