Hello. I bought my iMac 27 1T Fusion drive in 2013. Ot came installed with ML. Never upgraded the internal HD. I us external SSD for upgrades. So I'm using Mojave. Sometimes I boot from te internal HD and in 2021 only Safari works bad. Mail, Aperture, Lightroom, Office, etc works fine if they have been kept on the last version ML could accept. Replace Safari with Firefox Legacy and all will be fine. Just don't expect ML to be Mojave, Catalina and up. Keep in mind, you're working like you did in 2013. And believe me, the difference is not that much big. Maybe using iPhoto is the worst compromise. But that's all.