Try this:

REPLACE(accountx, '-', '.')

Just delete the rows.  If it's not that many rows, it's more work than it's worth to try to TRUNCATE.

I suspect (hope) the temp table is clustered first...

For this particular need, you just need to touch up your original script to bit to avoid unnecessary errors:

CREATE TABLE ##temp_fbl
(
DBName nvarchar(200),

CAST is more flexible than specifying a specific format.

Thus, I'd suggest TRY_CAST ... AS date to perhaps allow more data to be automatically converted to a date.

I suggest starting with your actual solution, not listing things you would not do first.  You can offer to explain why you wouldn't use other possible solutions if they're interested in...