Basically to start I feel it’s easier to pre-paid for credits (just lightning sats). But I would need a way to deduct from the total paid for a specific pubkey.. as a pubkey is effectively the payer. For example: I pre-pay 5000 sats (linked to my pubkey) I then pay 10/sat to publish my event to...

I think you're going to need a custom solution. It's easy enough to set up an LN invoicing system e.g. btcpayserver, but you will need a web interface for users to input their pubkey, validation, check whether invoice is paid, and keep a simple database of pubkey-balance records that you update as t...