Why this route works
Bangkok is not hard because the sights are far apart. It is hard because heat, traffic, and decision fatigue make normal sightseeing plans fall apart. This route protects the mornings, uses rail or river movement when those are better than roads, and puts recovery time in the middle of the trip instead of pretending every day can be full throttle.
Where to stay
For a first trip, choose a hotel that makes movement boring. Asok and Phrom Phong are the easiest Sukhumvit bases because the BTS is useful every day and the MRT is nearby. Silom and Sathorn work well if you want business-district calm, river access, and less nightlife noise. The Old City is atmospheric, but it is a weak base if you expect to use rail every day.
The main rule
Put outdoor temples before lunch, put errands or hotel recovery after lunch, then make the evening about food. Bangkok is far more enjoyable when you stop treating 14:00 as prime sightseeing time.