Have you tried to do a top up about 20-30min before nap time instead? Ideally, you'd like to do it an hr after the initial feed as that tends to help the snacking issue and that is when you will eventually do solids, but if he's not taking it then, it may not be the ideal for him. You can do it before the nap and just make sure it's a true top up, not a feed to sleep.
For how long have you been back at work? Could the extra NW/NF have to do with him waking more to BF from you and for that closeness? The only way to really get him to eat more during the day is to feed less at night. So, doing your plan and not feeding closer than 3hr apart, then stretching that out to 3.5/4hr, etc.
How did it go last night?