I would point out that the purpose of multitasking here is not to make you juggle (who wants that), but rather to give you a variety of things to do at any given time. I prefer to send out scouts on paths I set, and then just ignore them until I think of them 10 or 20 minutes later and check to see what they have discovered, for instance. And for defenses I mostly try to set it up so that those are also automated, except when sizable waves come. So then my attention is spent on the offensive raids, and dealing with the larger incoming AI waves, with a smattering of other quicker tasks (scouting, defenses/econ rebuilding, build queue adjustments) as needed. Generally I wouldn't expect that to be too overwhelming, but I guess it depends what you're looking for.
The more you set those activities end-to-end, however, or the more you structure it so that you're only having to worry about offense because the rest of the game is easy because of cheats, settings, the scenario, whatever, the more it is going to be a grind. In other words, this is a game that is about a whole bunch of different things other than just offensive raiding. What you seem to be saying is that you just want to focus on offensive raiding, but then it gets repetitive because all you're doing is offensive raiding.
I'm not trying to criticize here, I'm just trying to illustrate the way that the game is balanced, or why most people don't find it too grindy unless they play it with very specific settings. For instance, AI 10 is pretty much always a grind.
For the AI types, I'd suggest playing Random Easier/Moderate, since that gives a pretty good mix. The Harder ones can be quite difficult, and I don't play with those much, myself.
Really, the core of this game boils down to this: finding yourself pinched in some way, with some challenge that is not easy to solve, and then solving it. That could be on a logistical, strategic, or tactical level. Often it involves finding enough knoweldge, or finding a target to strike, or deciding how best to achieve your economic goals, or whatever. There are a hundred little challenges like that. If you play on a difficulty level that is too easy for you, though, then that all fades away. You're never challenged, so it starts feeling like you are just grinding ever outwards.
Decisions are only interesting if they come at an opportunity cost -- if everything is so easy for you that you can just slam through one world after the next and rebuild in between, with no thought of repercussions, then your decision space shrinks and the game becomes way less interesting. Playing at a difficulty level that challenges you will indeed extend the length of games, but rather than making it a longer grind that will generally (AI 10 and such excepted) instead just set you up with an increasing slate of interesting decisions. To me, it sounds like you aren't being challenged in short, and I think that's part of the crux of your problem.
On a related note, if you're just slamming through all the planets without much regard for what they are, then not only does that likely mean you're playing on too easy a difficulty for yourself at present, it means that you'll have a really hard time on higher difficulties. Haagenti and Eraser have made an art (in two different ways) out of keeping their territory small and doing longer-range raiding to just take a few enemy planets and to try to win the game in a very strategic, very non-grindy way. You might also enjoy playing a style like that, and it would also enable you to more easily handle a higher difficulty. Given your desire for detailed management of a "crusade" against the AI, I think this would also match your interests well.
