Severance Pay calculation

I need to setup severance pay on ERPNext which is paid when contract ends or after 5 years

The severance rules are:

-For each month worked, employee is paid at a rate of 1 day per month of service s(e.g. if employee worked 20 months then they are entitled to 20 days pay as severance). (Daily rate would be basic salary /22)
-After 5 years, the 60 months severance becomes due and going forward (more than 5 years) accumulates at 2 days for each month worked and will be paid at the end of the contract

I looked into the new gratuity rule in V13 and I don’t think it’s able to accommodate this. I see the experience is based on months worked (exact no of years or round off years worked)

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This will need to be reflected in payslip when it is processed. One third is tax free and two thirds will be taxed according to annual total income using the tax table.

I’m not certain, bit I believe the accumulated severance is a liability.

Should I do own doctype for this? can I customise the gratuity rule and gratuity function?