All other mt7620 seem to have at least one valid squashfs file. Does anybody know the reasons for that? The snapshots have an image again.
Since I don't know how to enter uboot or boot via tftp (I'm not sure this is actually supported for these devices). Is there any other way to get the initramfs-kernel to load?
Don't try to run the initramfs build. They are not usable for production because they run entirely in RAM and won't save your configuration to flash. They are useful for development and in some cases an initial boot to flash the production build by running sysupgrade in OpenWrt.
It is as @slh said, the build turned out too large to fit in the flash chip, so a squashfs file was not created. If you really want / need to run 19.04 you could make a custom build with some things removed. But most people should just stay with the 18.06.4.