I've never tried upgrading the OS of a phone, is there much point?

On the main brands you don't get the choice if you want to upgrade to the newer version or not, it'll come in with a security update and after the reboot you have a new Android version. Before that time you can tell the update to wait, with a maximum of 24 hours, before it will be installed. But update it will.

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What on earth could be the reasoning for this nonsense?

Plenty of zeroday leaks and bugs being found and fixed, which you do want. A lot of these bugs and leaks are being attacked and used by nefarious people. So the updates that come in will be the latest CVE updates, perhaps with a month delay. But it's better than having the bugs still in there like they then are in the older Android versions that no longer get updates and therefore are vulnerable.

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Yes, it's mostly up to the phone manufacturer if they want to release updates or not. Most cheap brands won't ever.
Therefore I said main brands. << Those will get Android 10. Mine luckily not, as I am not interested in it, but I did just the other day get the latest February 2020 update in with CVE fixes. So that'll be set to continue for now.

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Yes, that's possible. At the XDA-Developers website you can find a lot of custom ROMs. See https://www.xda-developers.com/what-is-custom-rom-android/ for more information.

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There haven't really been any significant changes since Android 5.
Android 4 brought copy/paste to the market, and Android 5 brought 64 bit to the market, that means support for more than 4GB of RAM. And that's pretty much it.

You get a phone, a browser, some apps.
The newer versions might support a 'dark mode' or 'night theme' or something in the OS.
But you're really not missing out.
Same old, in a different jacket.

On the main brands you don't get the choice if you want to upgrade to the newer version or not, it'll come in with a security update and after the reboot you have a new Android version. Before that time you can tell the update to wait, with a maximum of 24 hours, before it will be installed. But update it will.

I have a Fairphone which is certainly not a main brand. It gets updates but Google won't be releasing Android above 7 for the processor. I don't remember any problems with things being different when it did update the Android version. However it didn't try and do it on the fly, warning me that it wouldn't update without being connected to a charging source and not to interrupt during the process.

There haven't really been any significant changes since Android 5.

Which is in the eye of the beholder, of course.
https://www.computerworld.com/article/3235946/android-versions-a-living-history-from-1-0-to-today.html disagrees with you.

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Hello, may I ask whats the name of Your phone (I ve got sth like this - on dooge x5 max pro)
Send me message,, if you need, maybe I could help you to fix that)

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And I do wish they'd just call them by number. All these websites saying "if you have lollipop".... er what? I can't remember what bloody food it's labelled as, a number is so much easier.

It isn't just Android,

New Ubuntu, Focal Fossa comes out in two days time though most people hopefully will do as I do and refer to it as 20.04. Year and month of release seems the most sensible numbering system to me.