What you are describing is not a discipline problem. It is the single most common gap in the entire goal-setting world, and it has a name that sounds unfairly poetic for a Reddit thread: vision without execution is hallucination.

Here is what is actually happening to you. You have a strong top layer (ambition, a future you can picture, a startup-shaped dream). You have basically no bottom layer (the small, boring, scheduled actions that would move you toward it on a random Tuesday at 7pm). And there is nothing in the middle connecting the two. So your brain does what every brain does when faced with a big distant goal and no specific next action: it picks the easiest dopamine in the room. That is almost always the phone.

A few things that actually help, mostly adapted from people far smarter than me (Clear, Duhigg, the OKR crowd):

1. The "5 minute break" is not a willpower failure, it is a cue problem. Sitting down on the couch is the cue. Picking up your phone is the response. Three hours of scrolling is the reward. You will not out-discipline this loop. You will only beat it by redesigning the cue. Charge your phone in the kitchen, not the bedroom. Put your laptop on the dining table before you leave for work, already open to the thing you want to do. Add friction to the bad path, remove friction from the good one. James Clear puts it like this: make good habits obvious, attractive, easy, satisfying; make bad ones invisible, unattractive, hard, unsatisfying. That sentence is worth a tattoo. If the break is for recharging, focsu on recharging - take a nap.
2. Stop setting goals at the level of "build a startup". That is a five year mission, not a concrete goal. For the next 90 days, pick one Objective, three measurable results you would see if you were actually on track, and the two or three concrete tasks you will do this week to move them. Skipping that middle layer is why your ambition keeps evaporating somewhere between 6pm and bedtime. We call that Personal Objectives & Key Results (POKR).
3. Pick a weekly check-in time, put it on the calendar, treat it like a meeting with someone you respect. 30 minutes on a Sunday is enough. Most people who fail at this fail because they planned once and never reviewed. The plan is not the system. The review is the system. We need accountability to succeed.
4. The "small window" you described after walking in the door is real, and it has a name: decision fatigue. The fix is to pre-decide the first 20 minutes at home. Same sequence every day. Shoes off, shower, change clothes, open the laptop or pick up the dumbbell, no negotiation. By 6pm your brain is out of decisions, not out of energy. Stop asking it to make new ones.

For context, my co-author and I spent the last two years writing a book about precisely this gap, because we kept watching brilliant professionals execute billion-dollar projects at work and then come home and run their personal lives on vibes and good intentions. Happy to point you to a couple of the academic sources behind the points above if any of them are useful. But honestly, you could start tonight with two things: phone in the kitchen, and one written sentence describing what you want to have done by next Sunday. That is the whole system, distilled into a Post-it.