Train locally on your own machine if you have a GPU, or try using google colab if you don't. Google Colab has V100 which should fit larger batch sizes.

To be honest, given how limited the data set is and how small some of the segmentation masks are, I am not sure other architectures will be able to do any better than U-Net.

I would also try the nnU-Net which should give state-of-the-art performance, and so will give you a good idea of what's possible with the dataset that you have: https://github.com/MIC-DKFZ/nnUNet