you could also set up automatic syncing with dropbox for game saves..
but this will require some manual tweaking, cause damn games like to put those save files all over our home folders

You are probably being unreasonable. FTL doesn't use steam cloud and FTL is freakin' awesome. If you're refusing to play FTL, you're being unreasonable. ;)

You don't back anything up when you "update" (reinstall?) your OS?

Your savegames are typically stored in ~/.local/share . Back 'em up before you update. You don't have other files you'll be backing up anyway?

"The ability to delete games ... and keep my save files"

You already have this ability. Steam does not automatically delete your savegames when you uninstall a game. If you re-install the game, your saves will still be there, unless you manually deleted them.

"I know I could keep my games and OS on a different drive/partition"

Yes. You should do that.

Alternatively, you could just put /home on a separate partition, and reinstall your os rootfs separately, as needed. This is nice because you also keep your user's config files (KDE/GNOME settings, etc), and all your savegames. Steam saves your games executables and data in ~/.local/share/Steam/steamapps/common by default, so all your games stay installed too.

Keeping /home on a separate partition will save you lots of time in the long run. It helps keep your OS disposable. If something gets hosed, you can just nuke your OS, and keep all your files. You can even share your /home across multiple operating systems. For example, If you use Ubuntu, and want to try out Manjaro, you just need to create a small (20G should suffice) partition for your Manjaro root and install it there, and then just add your /home parition in fstab (or set up your /home using the partitioning tool at install time, just be careful not to format it!). All your files and games and user configs will be ready for you on your first login.

Setting this up is only a little bit of extra effort. You only have to do it once, and you will reap the benefits for years. It is easiest to do at install time, but if you want to do it after-the-fact, here's a guide:
https://help.ubuntu.com/community/Partitioning/Home/Moving

To share your savegames with your other computer, if they're on the same network, you can just share the relevant directories over SMB or NFS or whatever pleases you. If you mount the shares correctly, it will be seamless. If they're not on the same network, just copy your files with a flash drive and sneakernet.

I think it's unreasonable to avoid buying certain games because of lack of Steam cloud. Before we had Steam, you kept your saves on a memory card or something like that. I keep the backups of my game saves on flash drives or in the cloud. As Xpander said, you could simply sync the saves with the internet. That's up to you of course. When I update OS on a computer, I either back the saves up on a flash drive. (you could write a script for that even, and put it on the flash drive.) or I just forsake them. That's up to you though.