Why the warranty from Digitec is for HD WD 4TB only 2 years and for 8TB and 10TB lasts 5 years? (WD International is always 5 years)

I have a Synology 1517+ NAS equipped with 5 x 3Tb-WD Red (Raid 6). Two of my hard drive are dead. I would like to replace these two 3Tb-WD Red by two 6Tb-WD Red PRO. Will this configuration work knowing that there is a mix of WD Red and Red PRO with a mix of 3Tb and 6Tb capacity (I am fully aware about the fact that ony 3Tb of the two 6Tb-WD Red PRO will be considered by the NAS). I would also have 2 new 6Tb-WD Red PRO that work at 7200 rpm (with 256 Mb RAM) while the 3 remaining 3Tb-WD Red are working at 5400 rpm (with lower memory 128 Mb RAM).

Hello Digitec - Has the HDD already been written with data? 12 months warranty at Digitec and how much longer at the manufacturer?

What is included in the scope of delivery? Are screws and a SATA cable included? Possibly vibration-damped / rubberised screws?

In the specifications of the hard disk it says: "Transfer rate 750 MB/s". However, the following NAS (https://www.digitec.ch/de/s1/product/synology-ds920-0tb-netzwerkspeicher-13405988) states: "Max. Data throughput read & write at 225 MB/s. Is there any point at all in installing this hard drive or would the cheaper version with 175 MB/s also do the job? Main question: Are the 750 MB/s slowed down by the max. 225 MB/s of the NAS, i.e. can the full 750 MB/s be used at all?