Either you automate it away through sectors (then why does it exist...) or you require lots of extremely repetitive and shallow player activity on every planet
I think this is the ultimate issue. There may be other ways to make planet terrain interesting, but as far as pops and buildings go, there's no way around this, really. It either has to be automatable to a point, which will cause friction with those who think it's doing dumb things (and it probably will), or it has to be manual, which is tedious.
Maybe there are ways to make the placement of buildings and pops more strategic, such that a manual organization system is worth the tedium, but that's probably just going to be difficult. If it comes down to a numbers game of optimizing building and pop placements because of local resources or adjacency bonuses, then it boils down to what we have now where there's arguably only one correct solution anyway. A greater variety of buildings
helps with this, but I'm not sure it solves the fundamental problem. I guess there's a small element of strategic choice to it now, where for instance you might have a Dyson sphere and decide to overwrite energy tiles with mines for more minerals, but in the end that's not more interesting than just changing sliders to make a planet focus more on mining than energy generation. The terrain itself still doesn't
matter.
I'm not sure how to make the terrain more interesting in general. Maybe there's potential for ground combat, or for having multiple empires vie for control of the surface like PTTG?? mentioned, but in practice that sounds like it would just be too tedious itself for something that would probably come up pretty rarely, not to speak of the performance implications.
I think that the planets can be given more personality without the tile system or even detailed terrain. More detailed factions that group up on them would help, for example, especially if it influenced the planet's development in some way. Greater ability to specialize planets would help too, whatever that ends up meaning. The ecumenopolis teaser screenshot is kind of an example of what could be done, with planet wide commerce, mining, manufacturing or tourist (for unity I guess?) industries being things you could vie for.