This is overkill for your example 1, but the apple watch has an inactivity tracker. It reminds you to get up and stand for a minute every hour if it notices you

This is overkill for your example 1, but the apple watch has an inactivity tracker. It reminds you to get up and stand for a minute every hour if it notices you aren't moving. I've found it useful when I'm sitting around or working at a computer for long stretches. For your time to stop alert, the only thing I've found that helps is throwing everything on my outlook calendar - it gives alerts 15 minutes in advance of the appointment and that's enough to make me realize that some task is coming up and I need to prepare for it. I don't think I've seen an app per se that does exactly what you want