ETFs purchased in Year 1 totalled €10k, and those ETFs are now valued at €15k in Year 8, profit is €5k, thus €2,050 owed in tax.

This is the case only if you invest one lump sump. If you invest monthly, you need to calculate the gain on the 12 individual purchases throughout the year.

Is this Year 1 investment then forgotten about for another 8yrs? Until Year 16, and then if the ETF is now valued at €20k, €4,100 is owed?

You get a tax credit for the amount paid on the deemed disposal event at subsequent on the deemed disposal events or when you you exit the ETF.

With the same process repeating in each year, for example Year 2, €10k investment, worth €16k in Year 9, Tax liable €2,460?

Deemed disposal happens for every individual investment in an ETF, so you keep track of every purchase, not annual figures.

Finding it very hard to find information on how the long term investor actually deals with this situation.

Excel or an accountant. It's a significant amount of work.