Opencart - modification of the registration form

I would need to modify the registration and checkout page because I need to assign customers to a customer group when they register on the journal 3 themes on opencart.

There are a lot of these groups and it's cluttered. Standard registration: customer group is "radio" selection Modification:
1) select country
2) select a customer group based on country
3) assign the customer to a group

countries:
DE, AT, IT, HU

customer group in each country:
- private customer
- business customer non VAT payer
- business customer VAT payer

Note: customer group (private, VAT payer, non-VAT payer) is dependent on the country!
first select a country (SK, CZ, AT HU)
than select group (private, VAT payer, non-VAT payer)
the result is then 12 customer groups

Budget: 1500-2500

Thank you