Hello @Apsara! I've just launched my product StretchMinder -> https://bit.ly/31Y6ffy and from what I see it looks as if this would meet your needs.
Reading your story, it really sounded like I could have been the one that written that! I was suffering from constant neck, shoulder and back pains and just feeling constantly fatigued from being stuck behind my desk all day when working as a Software Engineer. I also have a habit of being way too involved in my tasks. Like you said all the activity timers out there required manual start, and for me I felt I required more movement in my life on top of just standing up. So even when I used something like the stand up functionality on my apple watch, I didn't know what movements to do because I've just exhausted my brain for the past few hours.
Which is the reason why I've added guided movement exercises which can be done during your breaks. I've also included breathing exercises because we also tend to take hamster sized breaths all day when sitting infront of a desk, and a walking exercise to get those steps in.
Would love for you to check it out and let me what you think! literally just launched this last week and looking to keep improving the product.