iMac firmware lock from stolen phone

So, my iMac got a firmware lock because someone stole my iPhone... here's what happened:

Years ago, when I was living in Japan, I bought a brand new iMac 27". A few years later, I moved to Brazil (where I currently live) and brought my iMac with me.

About two years ago at this point, someone stole my iPhone XR, got access to my account by exploiting a loophole (apparently they never had to get past Face ID, by putting my SIM card in another device and using the two factor authentication to send a new code to my phone number or something like that).

This loophole in the security allowed them to get access to my e-mail, bank accounts, and pretty much anything else that was attached to my main e-mail.

One of the first things they did, only minutes after taking my phone, was to go to the FindMy app and they erased all my devices connected to my Apple ID.

I lost everything I had on my iPad Pro (I'm a professional artist and lost a lot of work I had there).

But the worst by far was that they basically remotely formatted my iMac. Erasing everything from work files to personal things like years of pictures.

Not only that but my iMac was also put on a firmware lock, with a password I do not have.

I tired contacting Apple MANY times.

I called the customer support here in Brazil, talked to support on US. Went to an Apple Store and talked to people in person.

Spent many hours on the phone with them.

The best the could do was to tell me to take it to an authorised repair place with the receipt and they should be able to take care of it.

Miraculously, my mom (who is still living in Japan) was able to find the actual receipt (remember, I bought this in another country MANY years ago, 2014 I think), but once I took it to the repair place, they said that Apple did not accept my receipt.

I been through a crazy amount of stress because of this.

These computers are not cheap (especially here in Brazil) and my iMac is now essentially a very expensive, very useless paperweight.

If anybody can give me a light on what I COULD do, it would be very appreciated.

At this point I'm considering buying a device that forces the password.

I know it's not really legal but, I'm looking at it as me breaking into my own house cause someone changed the lock while I was out.