Based on my knowledge, IdleTimeout works for New-PSSessionOption cmdlet. You can try to use the parameter -PSSessionOption with the configuration:
New-PSSessionOption -IdleTimeout "a value in milliseconds"
If if still doesn't work, you can use EXO V2 module and the following command to connect:
Connect-ExchangeOnline -PSSessionOption IdleTimeout <value> -UserPrincipalName <upn> -ShowProgress $true
Please check this for more details: Connect to Exchange Online PowerShell.
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