I had previously investigated OGIMET (I think it's run from Spain) and it may be useful in the future, however the METARs are limited to 2005. Brazilian run REDEMET has API access to its OPMET https://www.redemet.aer.mil.br/?i=facilidades&p=api-redemet which is fantastic, but is limited to METARs from 11/2002, and is patchy for aerodromes outside of Brazil. I used to be able to scrape METARs from Weather Underground, but they've since disabled METARs from being displayed. For most international aerodromes METARs were available from 07/1996 at Weather Underground.
I'm keen to learn if there are any other regional OPMETs around which give good and long access.