Have you considered using bootcamp on the machine and updating to Windows 10 (which will give you till 14 october) or Window 11? It should work according to the internet and will put you on a train that is still getting security updates.
There is enormous amount of risk having an OS that is no longer getting security updates on the wide Internet.
OSes that get updates, are only vulnerable to zero day exploits that cost millions of dollars in the black market.
OSes that are no longer updated are vulnerable to known and disclosed issues, that leaves high risk around junior hackers just enlisting a computer to a DDoS botnet or worse still crypto style attacks where you have to send money to decrypt your computer.
We often underestimate how bad this can be, but there have been quite a few attacks we have seen over the past 10 years that can take out a computer simply by visiting a website.