Nice notebook! However, I think that it’s easier if you pass as_sequential=True in model creation. At least, gen-efficientnet-pytorch repo supports it. Then, you could use the standard fastai cnn_learner and pass cut=None. If I remember well, it works well with mobilenetV2 and EfficientNets.
>>> import geffnet
>>> m = geffnet.mixnet_l(pretrained=True, drop_rate=0.25, drop_connect_rate=0.2, as_sequential=True)
learn = cnn_learner(dls, m, cut=None, ...)