In companies, when they are building RAID it is common to order same hard drive model but from different vendors. Reason for this is to try to mix different series of hard drives, to ensure that if there is a problem with particular batch, not every drive dies at the same time. With different vendors (or same vendor who does this for you) you are ensuring a better spread of manufacturing dates and batches.
As for RAID 5, it is not recommended to have RAID 5 with drives bigger than 1TB due to high chance of URE (Unrecoverable Read Error). If you search internet, you will find different opinions, because URE is a statistical value specified by drive manufactures, which may or may not happen. But that doesn't mean that RAID 5 is not without risk. I've had multiple RAID 5 array's failing in enterprise environment where clients didn't follow best practices (replacing drives after few years, running RAID scrubbing, checking SMART, etc.). In 1 year I had 10 array's die (6 different clients) with drives from 320GB to 4TB in size and they all died during rebuild process. All clients were newly acquired clients from previous MSPs who simply didn't care. In few cases we were able to restore because we had backups, but not for all.
RAID 10 and 6 were much better for this, since both allowed for more drive's to die. I have yet to be in a situation where two drives die which hold same data (which would lead to loss). In general, I've used RAID 6 in arrays with 10+ drives, with size ranging from 6TB to 20TB without much issues.
For your use case, I would recommend RAID 6. You have 4 bays in your NAS so you can have same amount of storage just by adding 1 more drive. As others have said, RAID is not a backup. If you delete a file or get a crypto virus, RAID will not save you. So backup important files. For some clients (who wanted offsite backups on USB drives) we would configure backup tasks on Synology to copy the files to USB. Then the only "weak link" is you, because you need to remember to plug in the USB drive and do the backup. Otherwise there are cloud storages and Synology has apps/add-ins with which it can regularly backup your files to cloud of your choice.
