The 300 f4 will be an amazing lens and with the 1.4 converter all a wildlife photographer could dream about. I am already going to try the 40-150 2.8 on a safari and it works wonderfully with the same converter.
Apart from that, the 75 1.8 is an outstanding lens that I wish you all will have an opportunity to try!
The funny ting si that actual users of these lenses are very pleased with them.
As to faster lenses: i would heartily recommend the Metabones speed adapters, used with, e.g., Canon L-primes on MFT-bodies.
Kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....