I would honestly try some Easy Off oven cleaner. Make sure you rinse everything off with water after (maybe multiple times) to make sure there is no residue

If you prefer less fumes, maybe try a plastic scraper first to scrape the large caked on bits and then just put a tray filled with water and turn the oven on medium or high heat. Then scrape/wipe everything off. This would essentially be steam cleaning

Hmm, maybe adopt a kid to do the cleaning.