there are a bunch of things to consider which is why it seems so complicated. things like, do you prefer more storage or more live redundancy (aka how many disks do you want to lose before it can't recover)? There are also performance concerns you may or may not care about.
Hopefully someone familiar with QNAP's idiosyncrasies chimes in as it sometimes matters when making these choices.
If you don't care about all that, just want solid redundancy and don't need the most blistering performance raid5 is always a good go-to. You will hear a lot of back and forth on other mixes that work as well and they are worth considering if you care about any of the factors I've mentioned.
Also something to keep in mind, if you plan to do full cloud backups you can play with your arrangement a bit and figure out what you want. Simply rebuild your array and load the cloud backup. Its time consuming so only go there if you really want to try other configs.