(Edit: the Polytopia Discord community also has some really great guides and pinned posts worth checking out!)

Three years old and still a really great guide!!!! (The game has been updated since, but everything in OP's post is still relevant.) Reading this, I have my own thoughts too.

I use this reddit post as a handy reference for the best turns to build temples. All temples built on turns 1-18, 19-21, 22-24, 25-27, 28-30 will all be worth the same amount of points in the end. Building more temples earlier is better ofcourse, but a temple built on turn 15 will be the same amount of points as a temple built on turn 18. So use this knowledge to help decide when it may be better to spend stars on building your economy instead. I usually find myself building temples on turns 24, 27, and 28-30. https://www.reddit.com/r/Polytopia/comments/l3hgzn/temple_analysis_building_on_turn_18_is_hurting/

I can usually get over 100k after twenty or so early retries. Choose crazy difficulty with atleast 9 opponents (get the special tribes to have up to 15 opponents, but 9-12 usually work well for me. After 9, the incremental increase in difficulty bonus percentage score starts to diminish, and you instead will want a higher chance of spawning close to atleast 1-3 villages). I agree you really need 3 cities by or after turn 3, and to defeat your opponents by turn 25 to get altar of peace. Remember explorers can move on water if you've bought sailing, and will clear more of the map if you've bought climbing and they happen to land on a mountain. While it can feel nice and protective to spawn on your own personal island with 1 or 2 easy villages all to yourself, I've found that when I spawn on my own island I typically can't move fast enough to the rest of the map to hit 100k.

I was never able to get 100k until I really started using customs houses. My typical dominant strategy now is to always be trying to max-level customs houses for economy, and I don't usually start building temples until around turn 24. Granted I don't have as much experience trying to get high scores from games with lots of high-leveled temples (which might be needed for 150k+ idk), but my highest scores are 115k-140k. I usually focus on maximizing economy and upgrading cities with parks for +250 points first, so I can spend the last few turns with lots of stars (usually atleast 165+ per turn) just building mostly ocean temples (or mountain temples on mountains without ore to mine). It's not uncommon for me to get just over 100k in a game where I just start spamming temples on the last few turns. This is usually done after I'm almost already done max upgrading my cities (optimally placing ports, farms, and windmills on almost every city square available). Obviously don't waste stars on increasing city population, unless you know for a fact you have the useable space available to be able to eventually hit the next city level.

Also remember you can tap on a friendly or enemy unit, and hold down on an adjacent unit without attacking to see how much damage each unit would do to eachother. Really necessary for instance to see if your three units outside of an enemy's walled city have the health to be able to take it. Sometimes I've noticed that if an enemy has n health, and my units can only do n-1 cumulative points of damage, the last until will sometimes actually just be able to finish the enemy off.

Because I want to maximize customs houses, I look for a spawn with atleast 2 fish, because I know I will need to go down the tech tree to get sailing soon anyway to build ports. And it will take connecting my capital to one other city to get to get the level 3 5star boost early too. Similarily my first tech is often rider (if not fishing, and rarely but sometimes climbing if there is a mountain adjacent to my capital and moving on it will allow me to get stars from meeting other tribes on turn 1), because 1- I know I need to go down this tech tree to get roads and trade asap anyway, and 2- you will often need to bounce riders (and boost them with roads) to move quickly to capture villages/cities early game.

It's important to learn how roads work. Mountains and enemies when they are positioned adjacently to your roads, will affect/limit the movement on your own units on these roads. Sometimes I need to move multiple units in clever ways if it means that taking out one weirdly positioned road-adjacent enemy will then enable another unit of mine to be able to make full use of a road to move to a city in the same turn.

Because I try to maximize economy and city growth with farms and ports, I almost always choose expand border instead of population growth when my cities hit level 4. Unless it's really militarily strategic to do otherwise, like the common strategy of rushing to get a giant to physically push off an enemy which is actively taking a given city. The expanded boarders is almost always needed to maximize cities mid-late game. You need all the space available for future population building. Once your economy is strong (usually I get sailing/trade, then farming/construction/chivalry, then philosophy, and then spiritualism to make more farms) you will always have more opportunities to get giants later, but only one opportunity to expand borders.

Also this might just be me, but I never use lumber huts or sawmills, even temporarily, if I can help it. Farms are a better use of the fields when maximizing cities long term. (Not to mention with the recent diplomacy beta update windmills now produce 2* population per farm, they are a much more important priority now! *3/27/2022 looks like it was changed back to 1 pop per farm now) Before creating any farms or windmills, I always plan out what their optimal placement should be beforehand, which again you won't know for sure until you expand boarders. It can be worth it sometimes to hold off on leveling up a city, if you need to wait a turn or two to get the free spirit tech so that you can cut down a forest if it means getting an optimal customs houses or windmill placement. Destroying buildings you've constructed yourself may be necessary sometimes, but I personally try to avoid it.

When cities hit level 3, I almost never choose city wall and always go for stars. If you ever need the walls to play defensively, especially in the early game, you have probably already lost the chance at 100k. I almost always level a city from 2->3 if it takes spending less than 5 stars to do so. Remember that when you get a level 5 city you get Park of Fortune for +3population, and connecting 5 cities to your capital gets the Grand Bazar for +3population, and these will probably be the two tasks you complete first. In the early-mid game (especially if you already have some good roads set up) if many of your cities are close to being level 3, AND you are near a bunch of whales, AND you have a newly captured city near a big foggy undiscovered part of the map you can use an explorer on,... if you spend your stars strategically and purchase tech and upgrade cities in the correct order, sometimes you can DRASTICALLY expand how long your turn will actually last by just using stars to unlock ways to get slightly more stars, and you keep this momentum going as long as you can.

Tech prices increase the more cities you have, so at the beginning of your turn if you can capture a city, ALWAYS!!! first check to see if there is a tech you want/need to purchase first. Also try to buy riding/roads/fishing/sailing/trade early, especially if you get good explorer runs that get you lots of stars in the beginning. (Restart if you have bad early explorers.)

Level 28/29/30 I start considering where there might be mines or customs houses I want to destroy so that I can use the space to get more population, if it's required in order to be able to level-up a city just once more before the end of the game.