I appreciate that is a real frustration @LinzThompson, but I guess that’s the point of the Starter. HubSpot are good at what they do - including offering very good free/cheap solutions that are designed so that at some stage you will find a pain point so you upgrade.
If it’s any consolation, you can create a private app with the starter version which has a webhooks option. You could then send a webhook to PowerAutomate (included in your Office 365 licence) which can then use the API to do something clever - like look up the next number from a spreadsheet and update a property in HubSpot. I’ve not done it as most of our clients are Pro/Ent but might be interesting to solve for this if there was enough demand.
If you are interested in investing some time for us to try it, then get in touch at david.parry@wellmeadow.co.uk - but tbh, at some point you may end up upgrading to Pro for other reasons and then you’ll have it anyway.
Good Luck.