Planet

I was messing around with the fighter jets and bombers from the first game, so it's super low lol

I don't remember the highest, but there are ones like 8 too. If you fly over the boat's path with a plane that can carry multiple bombs or over 500kg, and has 20mm cannons, you can get a lot of kills.

Even with this many achievements, it's still SL16000... The SL acquisition gap between land, sea, and air is really severe, isn't it?

No, it's the sea. I think it also helps that I'm personally (probably) on the better side, but while achievements are relatively hard to get, you can normally get a ton of SLs with scores (especially Rank Ⅶ).

I feel like I've gone further before, but I want to punch myself for forgetting to take a screenshot.

The PR-1144 (the one in the screenshot) is recommended, but it was from a past season's gacha. So, your only options are to try your luck with the current gacha or grind for blueprints, which is quite a pain. (The PR-11540 is also recommended, but that's also a tough grind.) If collecting blueprints sounds like too much hassle, I don't have it myself, but the PR-1155 seems to have decent performance.

Also, for bombers, it seems like a good idea to use the terrain effectively to avoid anti-air guns and then go for a kamikaze-style attack once you get close.

Besides using terrain, flying just above the water surface (around 10m) seems to make you less likely to get shot down. Also, instead of targeting ships with strong anti-air capabilities, try to avoid their firing lines as much as possible, and you'll be even harder to shoot down.