New Synology NAS setup?

Hello, I have seen a few of your videos on YouTube that are great.
I'm looking to set up a new NAS. And have 2 questions about future upgrade options and performance.

I'm going for a 4 bay Synology DS920+
I'm also set on 4 x 4TB IronWolf Drives
I'm looking at a 2 disk fail tolerance with RAID 6 or Synology SHR-2 giving me 8TB usable space.

Will using SHR-2 with 4 identical drives give me the same speed performance as RAID 6? is it only when using different drives together you see a performance drop when using SHR-2?

If I go with RAID 6 can later upgrade the drives to say 4 x 8TB Drives, replacing 1 Drive every 6 months in the expectation of after all 4 bays are populated with the 8TB drives I can increase my usable space to 16TB?

Thanks.

If you have a 4-bay and you need the best performance with 2 drive redundancy then RAID10 will be the best choice. But if you do not relly on best performance as IOPS then there will be very little difference in which RAID to choose since you are limited toa 1GbE speeds anyway. Internal drives will operate at 200MB/s each.RAID6 will be as fast as SHR2. With RAID6 you will only get more capacity when you finish replacing all drives. SHR2 will add additional storage after replacing 2 drives.I hope this helps https://www.synology.com/en-uk/support/RAID_calculator