However, a fix like that is easy to describe but doubtless would get bogged down in ridiculous fiddly details if it were actually implemented. Saying 'just adjust the resource costs' sounds simple, until you try to do it.
Yes, that's why we don't want to go that route; trying to shoehorn two different games into the same general scenario doesn't have good results.
if you're expected to start with a lot of worlds and then lose them steadily until the timer runs out... that doesn't sound fun.
It might not be your thing, then
Though in my experience a successful defender game doesn't involve steadily losing planets the whole time, but rather holding your outer defensive line pretty solidly until something massive just smashes through and you figure out how good you are at disaster recovery. In a 45-minute 10-starting-planets (central planet and inner ring on a concentric map) game I managed to hold my two chokepoints pretty well (though increasingly tenuously) for the first 35 minutes or so, and then they busted the door down. But I'd held them off long enough and fought a pesky enough rolling retreat that they didn't reach my last world in time and I won.
And from what I hear it can be cheesed by just starting a huge map with all the planets as human worlds and then doing nothing until the timer runs down.
Several things are in place to make that less likely, but yes, I imagine if you play a 120-planet snake map and take all but one extreme end then you'll win automatically on a small enough timer. My suggestion (and that of the lobby tooltip, via the suggested number of planets) is to simply not do that, unless that's the kind of game you want to play for some reason (seeing how many planets you can actually keep against those uber punishing waves, etc).
Perhaps it shouldn't even be an option to start with one world. Or you should be able to start with a hugely increased number of resources, or auto-build times should be cut by 1/2 or more for this mode only, or something.
Have you tried taking a single world and a +300% handicap for yourself (and possibly a negative handicap for the AI, or some other adjustment like that)? That may not be as much change as you're looking for, though. I haven't tried it to see if it works.