I would be careful enlarging the hole to put in some kind of insert nut, helicoil, etc. Enlarging the hole might not leave you enough meat on the frame for a secure hold.

Simplest is best. You might be able to run a tap through there so a slightly longer bolt will catch enough for to hold. Or, as someone suggested, use a longer bolt with a nut. This is not a very elegant solution, it will look awful. But you can totally back out of it with nothing lost.

Another possibility might be to go up to a 6mm thread size. This might require drilling out the hole a bit more, but not nearly as much as an insert would require.

Be curious to know how you decide to handle this.