From what I understand, Apple removed the ability for TB2. In order to use an eGPU, some system files need to be patched [0].
I'm in a similar boat with a kext that enables unsupported Thunderbolt 3 docks.
I might have to use PurgeWrangler for an older iMac. Apparently, can keep it mostly enabled, but you need to mind updates and be ready to recover if your modifications are invalidated https://github.com/mayankk2308/purge-wrangler/issues/2#issue...