I use a simple 2.5" external usb drive connecter to my router as a shared drive on my home network. I also use it to run my router’s torrent download tool and get new content.
The external drive is not new and I worry that with the constant load it might fail one day. It’s also slow sometimes when watching content directly from it with kodi.
What would you suggest I replace it with? Would a 3.5" external usb drive be safer?
A single drive is always going to be a potential point of failure. Definitely make periodic backups to a different device if you don’t want to potentially lose everything on it without warning.
There are more complicated solutions available, such as a RAID array, but it sounds like you want to keep things relatively simple. In that case, I don’t think there will be a whole lot of difference between 2.5 and 3.5 inch drives – except that there are 3.5 inch drives designed for data center applications which may net you some extra reliability. You’d likely need to get such a drive and put it into your own external enclosure, though.
That said, there’s only so much you should expect to get out of a single drive connected via USB (relatively slower transfers, reliance on the bespoke external bay, potentially not getting warned if SMART status changes).