Still at the 'project' stage, but I'm working on a flatter BRM 'Grand' for September next year, 01/09/2022: Only about 3000m climbing instead of around 11000 and 8.25 AAA in the last edition! Staying east and well away from Wales, Cotswolds, Forest of Dean, obviously ... might get more than 12 starters and 9 finishers too.
There are a lot of tough 600km+ events - maybe my niche is something a little more benign?