Try https://dortania.github.io/OpenCore-Legacy-Patcher/ I recently used it on a 2014 5k iMac to install Sonoma (14.3) and it's been flawless in my experience - everything is buttery smooth as you'd expect.
I also installed on a 2015 Macbook (12"...horrible Intel mobile processor.) It works, but the CPU is just too underpowered to do basic stuff like web browsing and messaging...if you're impatient like me anyway ;)