I always thought it would be a better idea to use a ton of fighter-sized missile platforms so that building them was easy and losing them was no loss. If you have an ordinance transfer station hub or spaceport on the planet you don't even need since the planet can reload them. You'd need a separate Active Sensor platform to spot, but that can be fighter-sized as well.
It's a viable idea, yes...but "better" is a bit of a stretch. The smaller platforms would suffer from a fair amount of inefficiency, particularly as each one would need its own fire controls to control maybe 3-4 AMM launchers (size 1 launcher plus magazine = 100 tons, whether 3 or 4 depends on your chosen MFC size - and you do need a magazine, since ordnance reloading will take some time and you must be able to fend off a full load of ASMs) while a single 10,000-ton or larger platform could easily manage to have 2-4 MFCs in total controlling dozens to even 100+ launchers. In addition to the space saved on armor for a larger platform. Plus, a larger platform can get much better use out of skilled Tactical officers, double-dipping with both commander and CIC positions for a big boost to hit chances.
The advantage of fighter-size platforms of course is that they can be built without a shipyard, but if you have a big enough empire to spare the shipyard then the large platforms are generally going to perform better by a decent margin...not a huge margin, but enough to be noticeable.
I dunno, I think if I'm investing in shipyards for colony defense I'd rather have a few FAC or 3,000 ton yards to pump out potent, fast, but short ranged defense craft rather than a 20k ton immobile monstrosity. Investing a lot in static defenses that can't take the initiative seems like a waste by comparison, but I can see the sense in having a cloud of expendable missile launchers that can be pumped out and serviced by planetary facilities. And this way the enemy doesn't have one big, dumb floating object to shoot at but has to deal with 10s or even hundreds of small dumb floating objects that are hard to see and all mass their firepower together. The small, unarmored swarm is much more durable than the large fortress. The only true downside I see is how its consumes officers. If only automation was an option and we could have autonomous drone platforms, that'd be amazing.
Its not going to stop a determined enemy assault, but no mere planetary defenses should. The best defense against an enemies battle fleet is deploying your own battle fleet to intercept them. You should never rely on local defenses to do so and expect anything but a slight delay a determined enemy and your investment should reflect that imo.
I think a better alternative to a large, immobile defense platform would be a heavily armed and armored warship with very little invested in engines or range relative to its size. I believe these are called 'Monitors' and they seem like a much better alternative to what OP is suggesting as they can at least move on their own and might have a chance of evading slower missiles.