Wiz's defenses were tier 2, and using old weaponry by the time the Federation lead war came about (I think they were done before the first yust league war). They were speedbumps at best in the second.
His relatively new fortress was 4k strength. If he used tier 2 platforms with similar weaponry they'd have been much more powerful than the OG hug box.
A system with a well put together set of defences would be capable (IMO) of defending against a large fleet, but defenses will need to be constantly upgraded to keep pace. I think they can be upgraded but we'll see on release.
The cost is likely far too high to put them in every world, although that (imo) depends on how the sector mechanics play with defenses. Can you stick a fortress in a sector system and have it maintained by them? Will sectors build defenses if you give them a military focus?
Wiz effectively had his entire empire with no fixed defense. Against a more 'switched on' AI (or human) he could have lost the entire khersonia system, not just the two spaceports (which themselves are a big blow).
A standing fleet will always be needed, you can't be defending weak planets or non populated mining systems with fortresses.