Amount of item with tax included (gross)?

Myclientbase is the predecessor of Fusioninvoice and was also Open Source.
In the invoice itself, you won’t see that really.
It is just to enter a gross price . E.g. I have a „product“ which costs 4.49 per month including tax = 53.88 per year. If I enter the price without tax (3.77 rounded), the price per year is 53.83 including tax.

I have already seen other threads regarding this:
https://community2.invoiceplane.com/t/topic/5189
https://community2.invoiceplane.com/t/topic/7478

Is this so hard to implement? Because I’m thinking about implementing it myself (though I’m not really a developer). Could you tell me which files I need to change for that?
I would create a new database field called “item_tax_option”, 0 is without tax (default) and 1 is price included tax. Then in the frontend a new checkbox “Tax included”. An if tax included I have several price calculations to change. Would you tell me the files to be changed?