I'll start by saying - if you enjoy RTS games just buy it since there is more S in here than in 5 other games put together so just go for it...
Regarding SP - although, as you know, there is no campaign or story, there are loads of maps ranging from single solar system with handful of planets to huge ones that might take you day or two to complete... All this is backed up with simple yet rather well designed achievement system broken into dozen upon dozen bits that you should go through before reaching 100% (to give you an idea - i have been playing this game for months and am still no where near of reaching 100% achievement) So yes, there is plenty things to do in here... AI is exceptionally good too and it will keep you be busy for quite some time
Regarding your question on fleet management - it is genius! Simple as that really... You will pick it up very quickly and control dozens (hundreds!?) of ships in no time
Real gem, however, is in playing online vs human opponents - this part of the game is arguably the only one where developers are not reaching and delivering full potential of Sins... Yes, it is very good as is, but there are fair few 'holes' to put it simply which are preventing this game to truly shine online hence rather small online community.
Hope this helps