Have you decided on lights? I would go with the 24" fixture, I think you're talking yourself into going beyond a softie tank.

Either way you can have higher light corals right below the fixture, and lower light ones in the outer areas or farther down.
I wouldn't put an actual log in. If it's ceramic, sure. It'll get covered in good bacteria, yes. The lines in your sand look like the powerheads are moving sand around and making a shadow? The back of your tank will get covered in coralline algae, but you can put anything you like on the outside of it.
You don't need an ATO if you don't mind adding (fresh!) water to your tank as required. Your seawater isn't unstable, it's just that evapouration only removes fresh water and leaves the salt behind. So if it's hot where you are and you lose a gallon of water, your salinity goes way up.
Sure you can buy distilled water. I have an ro/di system and I cannot get it set up properly. Even if it was working, it wastes 3-4 times the water it purifies. (They all do this, it's a function of how the ro membrane works) If you live somewhere with water restrictions or pay a lot for water that will be a problem. I'm going to use the wastewater for my plants, we'll see how that goes. You still have to buy new membranes and filters and media, perhaps as often as every 6 months. Your city water might be great, or it could be 'oops all my inverts died' levels of bad. The conditioner doesn't remove all the bad stuff. You should be able to get a report on your water by googling around, or phone the water treatment plant and ask them.
I dip corals and just temp acclimate inverts. My tank was wiped out by dinoflagellates. In my case I think it was due to my dry rock rather than anything I bought/was given. I doubt quarantine would've caught it, but I'll never know. Don't look up dinos, you aren't going to get them and it will just freak you out unnecessarily.
Keep an aquarium log! If you test anything, write it down. Make general observations like 'what's with the streaks in my sand?' , or 'bought 3 turbo snails'. I made a calendar in Outlook (I have to have it for work, don't judge me

that's only for my tank. It's very handy to look back, and see what started when and general trends of alkalinity, ca etc. There are lots of apps, too, some of which make pretty graphs of numbers over time.
Buy a good magnet cleaner. Consider a floaty one, the yoga required for me to re-attach mine in a deep tank is ridiculous. It doesn't scrape well either.
Find or buy a long plastic stick. Handiest thing ever for seeing if that hermit is actually stuck, picking suspicious gunk, scooting the stupid magnet cleaner over when it detaches..
Look into macroalgae? Lots of super cool inverts but you should pick your fish first. Lots of fish think they're a nice snack.
Find your local reefers club. They'll know all about your water, where the good stores are, they'll have cheap frags/algae/used stuff for sale and are generally really helpful for panicky newbs.
hth
Ivy