Send each invoice to the person who should see it, by rules your team sets, instead of by whoever remembers.
Route by property, by amount, by expense account, or by a combination, to the approver those conditions call for.
A bill above someone's limit goes up a level automatically rather than sitting with someone who cannot sign it.
Each property can name who approves for it, so a manager sees their own properties' bills and nobody else's.
Add a one-off approver to a single bill when it needs a second opinion, without editing the rule.
An approver can send a bill back for more detail without rejecting it, and the submitter sees what was asked.
The approver is told when something needs them, by email and on their phone.
Every approval, rejection, and information request is timestamped and attributed.