God, so much support. I've been playing a game with Gotcha and a few others right now, and we've been taking our time building and farming - by no means is this a speedrun. The result is that we're up to our armpits in invasions, and they aren't challenges, they're interruptions. Goblins while farming or building means I have to go to center surface and clean up the inevitable mess. Solar Eclipses while organizing means I have to get out of the house, lest the expert-mode Mothrons and Reapers eat all the NPCs. Today we had a solar eclipse spawn in the middle of Moon Lord fight #7, and it wasn't even that threatening - just hideously irritating to have vampires and spheres flying at me while I'm trying to kite around his sweeping death ray. Maybe if there'd been a Mothron before he died, it would have been a problem.
And that's really important; this isn't about shirking a challenge. It's about beating the challenge so many times it no longer comes close to being classified as a challenge, and still being forced to participate when I want to do something else. Invasions are great fun at a certain point in the game and highly rewarding, but at the end of everything, I'd say the player has earned the right to skip them, the same way the player earns the right to fall off a sky island and not die or to instantly teleport themselves across the map as time goes on.
At the very least, I'd like to see random invasions, blood moons, and eclipses become much less common past a certain point in progression. I like them being frequent enough when the player still needs their loot, but after that? There's a window of relevance for each event, where before is too hard and after is pointless. Loosely speaking, here's the scoop:
*Slime Rain: Beginning of the game to Eater of Worlds/Brain of Cthulhu.
Blood Moon: Beginning of the game to Eater of Worlds/Brain of Cthulhu, then beginning of hardmode to first mech.
**Goblin Invasion: Relevant exactly once in pre-hardmode. Then beginning of hardmode to first mech.
**Pirate Invasion: Beginning of hardmode to post-mech, though player won't be able to fight them well (without cheesing) before first-mech/ready-for-first-mech gear. Additionally, they have many drops that are useful to the end of the game, and as such don't obsolete as badly as most other events.
Solar Eclipse: First mech to Cultist.
***Martian Invasion: Golem to Moon Lord.
*Slime rains aren't a problem for many reasons that don't apply to other events. They end after a certain number of enemy deaths, which can be achieved extremely quickly for the second half of the game. They go away on their own. The enemies they spawn are not very capable of hurting you or NPCs, so the rain can be ignored even if the player is exposed on the surface. The boss is threatening for early-game (if you don't know how to cheese it, which can be done as soon as the player has some rope and something that can hit at a distance), but doesn't spawn unless the player actively participates in the event.
**Pirates and goblins have the doubly unfortunate property of being absolute murder at the very beginning of hardmode - and like Gotcha says, you can't wait out the event like a moon or an eclipse. If you get one of these too early, you're stuck with hurling gravestones across your lawn until you eventually manage to fit in enough stabs-between-deaths to get it over with. These need either the automatic despawn or some way of making them unable to appear before a certain time. Post-first-mech is too late, so maybe the first two days of hardmode, starting the 4:30 after the player defeats the Wall of Flesh?
***I have no problems with Martian invasions because they don't occur randomly. Having to go fetch them yourself works great in practice and leaves me with a general positive feeling towards this event versus a highly negative one for most of the others on this list. In addition, it drops two mounts that are the best at what they provide and an exclusive decorative block, so like pirates and unlike the rest on this list, a player will have reason to fight them past the obsoletion of their weapon tier.
That's a lot of bullets, but getting to the point that ties them together, another thing I'd propose is keeping the current spawn chance in those windows (exact points of progression need fine tuning, these are just suggestions), and then tanking the natural spawn rate after their obsoletion - at most, a fifth of what it is now. Give Blood Moons a manual summon item and I'd call this good to go.
Alternatively, I'd take a post-Moon Lord item that, when placed in a world, blocks invasions/celestial events from occurring randomly. Easy enough and allows customization of gameplay styles.
It would be nice to be able to build in a hardmode world without constantly getting interrupted. I don't really care how that's achieved... I just want to see it happen.