The problem on Deity is it's always about exploiting the AI rather than doing anything on your own. I think it's likely that on that level, you will simply continue spamming units and killing AIs, leaving them one city so that you can farm for gold (by selling surplus luxuries, which they will still buy) and settlers. In fact, this is probably already true on Emperor. As for whether ICS is optimal, this really depends on what victory condition you go for. Boosting pop seems the go-to strategy for getting scientific and civic progress, and new cities will do that for you. I think it's possible to build at least one or two additional districts in those small cities, especially if you leverage chopping and harvesting (combine with overflow bug if that is your cup of tea), and then you get a situation that is very similar to the initial CivV build where you only created cities to get a library and university so you can spam great scientists. While the latter aren't that useful anymore, population and possibly building campus and theatre is still the best way to get science, and we should not forget that you get two free pop that don't need amenities or additional housing in every city. Of course science in itself is only really useful if you go for the rocket.