It may just be that your letdown is slower in the evening, not that you have less milk at that time. But you could try breast compression as your baby feeds, to help get the milk out of the reservoirs.
Regarding the expressing while away, I think it probably depends how young your baby is and how long you are going away for. If they are under 6 months I would certainly carry on expressing so as not to mess up the balance between supply and demand too much. And I would probably do it more often than once a day.
I think if you express regularly then your breasts get used to this regular 'demand' (almost like an extra feed) so that's probably why your breasts feel full in the hours after 6 pm.
With ds2 I generally only express when he's away from me and missing a 'real' feed, in which case I get plenty of milk - pretty much a whole feed's worth. But with ds1 I did a lot more expressing in between feeds and sometimes it was only an ounce or two so I had to do it several times to get enough for one feed.
Regarding timing of expressing, I think you have to experiment to find what suits you. Some people say to express immediately after a feed as there's always some milk left in your breasts (and it will be fattier hindmilk which is good for supplementing with). I personally find it really difficult to get any milk by pumping immediately after a feed. So my rule of thumb with ds1 was to wait about an hour after a feed, as I didn't expect him to feed again for a while. But I know it depends on the baby (ds2 feeds far more frequently than ds1 did).
On the occasions when they have surprised me and want to feed earlier, quite soon after pumping, I do feel like my breasts weren't producing milk fast enough for them and we both got frustrated. There's a good article about how milk production works here on kellymom.com . Based on this information, I guess that it was probably just that the letdown was slower just after pumping, so my kids had to work harder for the milk. But I'd try to leave a reasonable gap for both your sakes!
This is all purely based on my experience and the information I have read, a mnetter with more experience/training may give better advice, but I hope this helps a little.