Never had this happen, and I've carried laptops since the mid 90's, and they've always been plugged in most of the time.
Get to office, plug on, get home, plug in and sit overnight in the charger with no use.
I've seen a few expanded batteries, but that's across the hundreds of laptops in my support circle. It's very rare.
Every laptop I've had in the last 5 years has battery protection built in anyway. I'm running 2 laptops from 2019 that have it.
Though you do make a good point, something to figure out if your laptop does this. And to keep an eye on the batteries anyway (like check battery health quarterly), and replace if it gets down significantly (I replace mine at 70% health).
USB isn't good for RAID, it's unstable.
Do you currently have more than 8 or 12TB of data? Because you can buy drives that size today, no need for RAID under those capacities.
I recently purchased an 8TB drive for ~$100 on Amazon. Yes, it's used, but comes with a 3 year warranty. I'm fine with that warranty length, as drives don't last forever, and I'll be replacing drives due to growth anyway.
Don't overlook RAID 1 - mirroring. With large enough drives this is a viable first step to some redundancy (though it's really intended more for failover). Simply replicating your data locally to multiple drives, and backing it up offsite should give a lot of redundancy.
The big challenge with local redundancy is that it's not backup, so replicated bad changes can wreck all local copies. Backup, however, gives you multiple copies of data and incremental changes (if configured that way).