I generally create 4500T survey ships early in the game.  With a big, low-power civilian engine (40HS or so and 0.3 power), and a cheap civilian jump drive (even on the grav survey ship) these can carry enough fuel for ten years of surveying.  Gravsurvey ships get some engineering spaces and geosurvey get fuel and/or sensors to bring them up to the jump drive's maximum size.  They're boring ships, perhaps, but are cheap to build and require very little micromanagement.  As tech advances, you can give them more speed or, alternatively, even more fuel efficiency and an extended mission range.

I am curious: Have you ever noticed the game becoming slower with all those sensors around? I have recently started to heavily mine some jump points, with mines of course having sensors (though passive only), and I have some suspicion that it made the game intervals somewhat slower. I don't know for sure though, - sometimes it is slower, sometimes everything seems normal.

Yes... if you use the naval organizational tab to select and order your units you can keep quite complex fleets and still remain sane.

I always give capital ships (destroyers and up) a 250-1000t hangars where I can fit a plethora of recon, scout and or rescue shuttle crafts. Since I extensively use the naval organizational tab this is possible to manage.

You then make extensive use of the escort and hierarchy system to order them around without too much hassle.

In my current game im using 1K Ton dual purpose scouts. with a 9K ton Carrier vessel that carries two of these vessels each. All the Carrier does is act as a Tender for the smaller vessels with a Jump Drive fitted. Sensor platforms could work but logistically it would be a nightmare for you to give all the orders and such to move them around.

I have played two games with specialized sensor jump scouts as exploration plan, who ended up being 5kt, so not that big at all (though they were separated into geo and grav, yet could also have been even smaller as they carried more than one).
In every other game I used exclusively carriers with first 500 ton shuttles, but later shifting to 1kt scouts, as range, maintenance and speed on 500 tons with 250 tons already occupied by the sensor turned out too impractical. Usually 2-4 "drones" of each type here.

The idea to have a modular exploration platform tugged on by civils is pretty good though. I also used 'modules' combined with large civil tugs often (for militar: troop drops and active sensors so far), yet never for exploration yet. I think you should go with it.

I was tinkering with the current game I'm running for an AAR, trying to design a multi-mission jump-capable ship with geo and grav sensors, but the sheer tonnage requirements for it dictated I find some way of lowering the size so that I don't have to spend nearly a hundred years expanding my 2000-ton naval yard with an extra 10,000 ton capacity.

While ruminating on the injustice of grav sensors being military, I started to wonder; what if rather than building mobile grav and geo surveyors, you built stationary platforms, giving each a pair of whatever sensors you need on them, and removing anything unnecessary? To me it seems viable since I have tugs under construction, but what do others think?