I married into toaster moguls. When they sold out in '97, domestic toasters had been infeasible for years already. And this was for a company where all the knowledge, equipment, and facilities had beed paid for many decades before. (They invented the electric pop-up toaster for certain definitions of electric pop-up toaster.)

Toasters are refined brilliance, if done right. The concept of "done" is computed using an analog computer programmed by human experts. (Ok, its usually a bimetal strip but it is placed so that the cooling of the moist bread keeps it from going off and your lighter-darker input is biasing when it considers the toast done.)

Tear apart some toasters. There won't be anything in a modern cheap toaster that isn't absolutely required. Ask yourself why everything is the way it is.

Research the UL requirements. I have the corporate 2 pound copper ball that had to be dropped on things from prescribed heights and not cause malfunction. Make sure you can hit this targets with what you think you can build. Also check the CE, they might have more modern rules.

Be ready for litigation. Toasters catch fire. The toaster moguls were horrified whenever they saw someone's toaster under a cabinet. Decades after selling the business they were still being sued by mesothelioma suits for things like a repairman that got lung cancer and repaired home appliances, so he probably might have worked on one of their 1920's models with asbestos insulation. Don't let it stop you, but put the backup insurance into the expenses.