I can not say how big ML companies do it, but from personal experience of training vision models, you can absolutely reuse the weights of barely related architectures (add more layers, switch between different normalization layers, switch between separable/full convolution, change activation functions, etc.). Even if the shapes of the weights do not match, just do what you have to do to make them fit (repeat or crop). Of course the models will not work right away, but training will go much faster. I usually get over 10 times faster convergence that way.