Buy now and then regret later. I'm leaning more towards iPhone 8 because iPhone X is over my budget.

If you seriously interested with iPhone 8 Plus then I suggest go for the premium iPhone X.

IPhone 6 will facing a lot battery drain issue.
There is not lasting anymore.

If continue with iPhone, could be don't think about upgrade so soon for iPhone 8.

No booking , is it ready stock?

I sad now, my ip7 could not sell well.
Feel like nobody want.

True tone colour it's normal when you turn on true tune, the function is about the same as night shift. It's depending on the temperature and brightness of your immediate environment.

about ur apps on previous phone you can backup to your itunes, but latest itunes won't backup the app's IPA copy to your PC already.
After you done restoring your phone, those apps will auto download back the apps that your previous phone have. If you want to keep your whatsapp history, you have to backup in ur whatsapp's app to icloud first before you start to restore your current phone. Once you done restoring, the first time you open your whatsapp it will prompt a message to ask if you wanted to restore back your history.

hope it helps to solve your problem 

Are both phones having the same apple id? If yes, then all you need to do is to do a full backup of the iphone 5 in the latest iTunes. Make sure that you pick the backup with password option. What this does is it also backs up all the logins and passwords too. When thats finished, put in the ip8 and then at the main page of itunes, do a restore and pick the ip5 backup. You will be prompted for a password to proceed. Then starts the restore. All the data, settings, photos and apps etc will be put on the new ip8. When It has finished installing the files. Let the phone download all the latest versions of the apps. Its completed when all icons are no longer grayed out.

For the screen brightness, settings>general>accessibility>display accomodations>auto brightness - un tick this button. Then go into colour filters. Here can adjust the colour tint from all three prime colour to your liking. Tick the colour tint, play around with the intensity, you can see the difference.

Hope Jumpstat's replied can solve your problem 

latest itunes will only restore the user data only, not the app file itself. it will send the user data to the phone and then tell the phone to get the app from iOS App Store through the phone's connection.

to avoid this, you can search for iTunes 12.6.3 on support.apple.com. this iTunes is the last version to have the app syncing feature. as a bonus, apple will not show you any further update to iTunes if you install this version

32bit app is not supported for iOS 11 already. It might due to the app doesn't meet apple's requirement & removed from appstore or the app haven't updated to support 64bit.

Do a backup with password of your old iphone.
Then do a restore to your new iphone.

you are still able to download them back to your phone

go to Apps Store > Updates > Your Photo (under the battery sign) > Purchased > My Purchases > start searching!!

as long as iOS 11 still supports them, you're ok

that's strange, i am still able to look for it! well, maybe the developer removed it?

I think most 6 and 7 user wont be buying it, cause if i were one of them, i will wait another year.
Then again, those who willing to change will probably waiting for x.
So 8 will be cold in stock for sometime i guess.