Just got done a Galactic Warfare campaign where on the 3rd map I faced double commanders and one of them raced to the secondary planet. By the time I arrived with a landing force, it was too late. Every landing party was wiped out by bombers within seconds. I tried coming in with AA bots, but even those were wiped out. Could I have won? Probably. With 50+ loaders dropping a huge wall of AA bots and fabricators I might have been able to build a beachhead and gotten up a teleporter. But you know what? That's not fun. Loading and unloading orbital loaders is about as fun as it was in the very early days of development - not fun at all.
Spinners are better vs bombers as long as they are moving because they take more hits to kill. Stingers are glass cannons, spinners offer survivability.
Sounds like your landing force wasn't big enough. Your "could have won" scenario was the correct landing option. Did you know you could use area commands for the transport? You can load 100+ units with one click. Here's a tutorial – the commands are the same for the Astraeus as the Pelican. So moving 100+ units for an invasion is actually really easy. The way I invade has worked 100% of the time. Build 100+ transports, fill half of them with anti-air units, the other half with fabbers, preferably with a few advanced fabbers. As soon as the units land, build a teleporter, and then defensive structures, and then factories. As soon as the teleporter is built, link it, and swarm units through.