I have a HP Inspiron 15 3521 that, after years of steady use now

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3. How old is the laptop? Does it have a hard drive or a SSD?

I searched the model number of your laptop. From the results, your machine appears to be a six-year old Dell, not an HP, and it came with a hard drive, not a SSD. Since you already re-installed Windows 10, and that didn't solve the problem, the next thing to do is to replace the hard drive, and install a fresh copy of Windows 10 from a USB flash drive. You can download that from Microsoft's website, but you'll need a working Windows computer to do that.

Hard drives get slower and slower as they age, and they can even struggle to boot up an operating system. While you upgrade the drive to a SSD, consider increasing the RAM as well at the same time.

I have a seven-year old Acer laptop that was running really, really slow. There were instances where the laptop would take 3-5 minutes just to boot up. I replaced the hard drive with a SSD, put in a fresh install of Windows 10, and now it runs much, much better and faster.

SSDs are now so inexpensive compared to what they cost even two years ago, it really makes no sense to use hard drives.