A: Hi see answers below:

And here is roadmap:
https://www.usetabby.com/roadmap/

-Customize the invoice number on client invoices - Answer: We can do this

-Have a date field on items added to invoices - Answer: We can add this

-Ability to download the invoice in PDF format. I have found that when emailing clients directly, they either don't receive it from systems because it goes to SPAM, or don't recognize it because they expect and want an email from me with the invoice attached.
Answer: PDF download of invoices will be available in a few days

-Hide unnecessary fields on invoices (I never show a tax rate or have an amount with tax) Answer: We can add this

-Group invoices into projects by client. - This is something will take little more time, but we can add as a feature request in next 30 days.

For items I mentioned we can add, give us until end of the week to add, if not sooner.