Maybe Armored Troopers Votoms? Haven't watched it myself but I hear it's more grounded than Gundam.
I don't think Japan really goes for sci-fi, anyway. There are a lot of cool sci-fi concepts, but as far as I know, I can't think of anything in the mainstream like Star Trek, where really about 40% of any given episode is quiet discussion on the scientific forces at work. Of course, the science may not always check out, but that's where the 'fiction' element of sci-fi comes in. You can really just have fun exploring the concepts.
As far as Gundam goes, I feel like 00 had the most sci-fi feel to it. This sentient computer program carrying on the will of a genius dead* German scientist, using this other-worldly technology. Artificial beings and aliens, to boot. I always loved how the Gundams had this sinister, other-worldly aura about them. Like, they were something that shouldn't exist, even in that world.
Couple it with an earlier 'future' than we've seen in other series and it really works. Season 2 was not as good, yeah, but still pretty solid show overall. In my opinion, unless some of the characters were little kids, time-skips usually fall flat.
My question is this: Did Kira even have any military training? How could he best Yzak, a young man with extensive military officer training? Programming a suit doesn't give you situational or tactical know-how. Athrun is an engineer and Kira is a programmer... how are they so much better at fighting than seasoned adult pilots? They are just 16.
OH WAIT... and I'm being serious here, I just remembered that Kira was an test-tube super soldier. He probably had all the combat shit programmed into his DNA, didn't he?