There is no easy or readily available option for this problem.
In my professional opinion, you should consider replacing this frame.
However, there are options if you really want to keep using this frame.
- Ditch hollowtech and use a threadless square taper bb.
- Bond the shimano hollowtech cup to the frame.
- Weld and re-thread
Threadless BB thread to eachother and just press onto the frame. These work well for a while, but if you're not super careful, they can (slowly) further damage your frame until it's completely unusable.
Shimano hollowtech cups have a bearing pressed onto them, you can actually replace the bearings of the cups instead of buying whole new cups. This usually doesn't happen because the bb cups are relatively cheap and it does require some skill and non common tools for the job.
If you bond the cup to the frame (making sure it's aligned properly), theoretically, you will just have a "pressfit" frame. You can then use the cup and replace the bearing when it wears out.
Video demonstrating the bearings inside the cup
Finally, unless you have a welder or know someone with a welder who is willing to help you, it will probably cost more to fix this than to get a new frame. The bb thread cutting tool alone will cost more than the frame, and I would be extremely surpised if a bike mechanic is willing to use theirs for such a job. Also, the heat from welding is very likely to bend the frame and result in misalignment. This option is completely out in my opinion.