@djcz - I have been running VMWare Fusion since version 6 through 11, on all kinds of Macs, from Snow Leopard to Mojave. Never, and I'm not lying, never had an 'Internal Error' on startup in all these years, different versions of Fusion, different versions of macOS, dozens of different Mac models (personal, work, family and friends machines). I don't get the 'Allow apps.....' warning either. For guest virtual machines, I have installed and run every version of Windows from 3.0 to 10, desktop and server, every version of macOS from Snow Leopard to Mojave, numerous Linux distros, even DOS 6.22 and old IBM OS/2. In all these combinations, never an Internal Error, ever.
I'm not dismissing the fact that you are having issues, and that is unfortunate, but I don't believe these are systemic issues, but are specific to your environment. Because in reality, if these issues were systemic, then MANY more users would be experiencing the same, and we'd hear about it, more importantly VMWare would likely be able to reproduce it and thus fix it.
Some suggestions before you give up on Fusion...., create a new partition on your drive or get an external drive, and do a fresh install of whatever macOS you use on the new partition or external drive. Your 'regular' working system won't be at risk at all. AFter the fresh install of macOS, don't install any other apps, add ons, tweaks etc, don't change or add anything, just install Fusion, that's it, and load a saved VM image. If no errors, then something about your everyday working system may be causing the errors. If you get the same errors, then possibly the VM image(s) you use are corrupted. To confirm that, install a new one virtual machine, like a copy of macOS or Windows from scratch. Don't worry about activation, you just want to test if it runs. If no issues, then that suggests your working images might be corrupted, either literally in the file image itself, or internally in the operating system running within the VM.
These are just suggestions, peace, and good luck.