|I could see there being some room for granularity, but if you make everyone 5:1 at a baseline, this makes no sense, as you also made everyone 1:5 on defense, meaning the baseline would be...1:1 (5:5 is equal to 1:1).|
It also diminishes the Lizard and Fascist advantages MAJORLY, and Fascists in particular are already not high on the advantage points to begin with (I believe they're second or third lowest, and that's with having two Glory Device ships that both give them 35 advantage points - even though the Nefarious should probably be 25 or 30 instead.)
Your main issue seems to be that Lizard attack and/or defense is too high, so you could just as easily go with a 25:10 for the Lizards and call it a day.
You can make arguments for some others (I remember playing a 3.5 game where Birdmen were set to 5:1 attack due to being Romulans/Vulcans), but that only works if it's still less than half the races.
When everyone has a ground combat advantage, no one has a ground combat advantage. It's like the saying "When everyone is special/a winner, no one is really special/a winner."
And if you tinker with that, you have to be aware of everything else that changes. The REASON Lizards have such a strong advantage (and the Lizard Class Cruiser) is BECAUSE they have one of the weakest ship line-ups in the game, even with their 150% shield health bonus. They simply can't take out things that are overly fortified. And even WITH that advantage, they are still considered a weak race late game due to their lack of large ships and that 30:1 becomes a lot less late game when planet populations are 10,000s clans and lots of defense outposts to boost their defense multiplier.
Likewise, Fascists are also considered an early-game race (though they do have late-game capabilities via Poppers and Pillage+Planetary Immunity).
Further, all this equalizing doesn't really change anything except for the Rebels and maybe the Birds anyway, since as you noted, unless ships have planetary immunity, planet captures are almost always done through ship-to-planet combat, not ground combat anyway.
So again, I see what you're trying to do. And I think there's some room in the game for more variation (e.g. Feds or maybe Robots are good at defense so 1:5, or that Bird flying ships and jumping out in armored bird suits actually DOES give a minor 5:1 attack advantage by "scaring the pants" off the local populations...), but when you get to basically everyone having an advantage, it basically negates all the advantages entirely, to the point of not even having them.
Also: Do keep in mind that defense outposts affect your defense multiplier. I forget what the number is exactly, but if you have lots of defense posts at your planet, the Lizards DON'T get that full 30:1 advantage. By having defense posts for a mere 3x defense multiplier, you cut that down to 10:1 already. Late game, Lizards are having to try to sneak in and drop from Super Transport Freighters, and that doesn't usually end well unless they have a Bird ally changing the FCs away from ATT/NUK. (Birds also are considered an "early game" race - the only cloaking race that isn't is Privateers.)