Help needed b/w 4TB WD Red Pro / WD Gold / Seagate Ironwolf Pro?

Hello,
I currently own a QNAP TS-453Mini 8GB (original version) w/ 4x 4TB WD Red Pro (64MB cache) HDD's (all bought in 2016)
1 of HDD's failed this week and I'm looking to replace it ASAP.
I was looking at the 4TB WD Red Pro vs the 4TB WD Gold vs the 4TB Seagate Ironwolf Pro HDD's.
I'd need some advice on the following:
- Can I mix HDD brands (i.e. 2x Seagate + 2x WD Red Pros)
- Can I mix HDD's with different cache sizes? (Red Pro 64MB vs Gold 256 MB, or even Ironwolf Pro 128MB vs Red Pro 64MB, and so on)
- Considering the recent SRM issues, are the WD Gold and Ironwolf models bound to be affected?
- Is the WD Gold worth it for a 4bay NAS?
All three drives are nearly at the same price right now, so that's not an issue. It's more on reliability.
I replaced one of my drives 1.5 yrs ago already and now a 2nd one failed this week. That's 2 of the original WD Red Pro's in less than 4 years. Add the emerging SRM issues affecting WD 2TB-6TB HDD's...
Thanks in advance!

Replacing a single drive to a WD Gold would not make much sense. But you can do it if there is nor WD Pro replacement drive available for sale any more. Mixing different cache cache drive would not effect things. Cache is used for HDD internal caching operations. RAID doesn't need it. Pro and Gold drives are not effected by SMR scandal. Only WD Red 2/3/4/6 TB. You can read more here:https://nascompares.com/answer/how-to-tell-a-difference-between-dm-smr-and-non-smr-cmr-drives-hdd-compare/I hope this helps.