Choosing correct RAID then also expanding individual HDD size in either RAID 5 or RAID 10

To preface, I am very new to RAID as I currently have my EX4100 in Spanning and now want redundancy.

My current setup is
Drive Bay 1- 6 TB
Drive Bay 2- 6 TB
Drive Bay 3 - 12 TB (not yet configured)
Drive Bay 4 - empty

I am currently moving all my data to external HDD so I can create the RAID then copy all data back to the NAS, this is a process I don’t want to do again.

I understand RAID 10 provides the best redundancy So thats prob the path that I’ll go.

But my question is relatively the same for both setups.

At this stage i am looking at puttingvanother WD red 12tb in bay 4. But as I am shooting a fair bit of 4k video and the megapixel count of cameras get larger, as does their RAW file size, I know I’ll fill up these hard drives pretty quick (especially if I go raid 10). Which will draw my attention back to drive bays 1 and 2

If in the future I upgrade them to say a 12tb each but replace the drives 1 at a time, will the data be automatically moved to the new one. E.g. I upgrade drive 1 to a 12tb but leave drive 2 in, will the data from drive 2 be automatically written onto drive 1 without me having to do it manually…and then repeat when I replace drive 2. And once both replaced I’ll have an extra 6tb to play with (that being the delta between the old drive size and the new drive).

Same question around RAID 5, if I replace and upgrade the size of drive bay 1 and 2 in the future will the data that was in the old smaller drives be written onto the newer larger ones and give me additional space to play with?

Knowing these answers will go a long way to determining the way I go ahead

Thanks in advance for all your help and I hope my question and dilemma makes sense