Two years of daily espresso and I have never done any maintenance, what actually kills these machines?

On descaling, and the thing that actually determines everything: what water you have been putting in it.

In a hard water area, two years of daily use with untreated tap water means there will be significant scale. In a soft area with filtered water, possibly very little.

So find out your water hardness first: your supplier publishes it, and cheap test strips exist. That tells you both how urgent this is and how often you will need to do it in future, which is the number nobody can give you generically.

On doing it:

Use a descaler intended for coffee machines, and follow your machine's own procedure. Some machines must not be descaled with certain acids, and some have components that are damaged by it - this is why the instructions differ and why the manufacturer's version is the one to follow.

Expect the first one after two years to be unpleasant, and possibly to need two passes.

Then prevent it. Filtered or softened water for the machine from now on. This is far more effective than descaling more often, and in a hard area it is the difference between a machine that lasts a decade and one that lasts four years.

If the machine has been struggling to reach temperature or the flow has dropped, that is scale talking and it is already well advanced.

Two separate jobs, two different problems, and you have skipped both.

Backflushing, removes coffee oils and grounds from the group head. Oils go rancid and that is very likely your slightly worse shots. Coffee is fat, and two years of it baked into the shower screen and the valve tastes exactly like old fat.

If your machine has a three-way valve, you use a blind basket and a small amount of detergent made for the purpose. If it does not, you rinse with water and clean the screen by hand. Check which yours is before you start, this is the one place you can genuinely do damage by following the wrong instructions.

Roughly: water backflush daily, detergent weekly, and take the shower screen off and soak it monthly.

Descaling, removes mineral scale from the boiler and pipes. This is the one that kills machines. Scale insulates the heating element and blocks narrow passages, and the failure is expensive.

Frequency depends entirely on your water, which is the next answer.

Start with the backflush and the screen. That is an hour, it is the reversible half, and it will probably explain the taste.

The third category, which nobody mentions until something leaks: the rubber parts wear out and they are cheap.

The group gasket, the seal the portafilter locks against. It hardens and shrinks over a year or two. Symptoms are having to turn the handle further round than you used to, and water seeping around the edge during a shot. Replacing it is a few pounds and about ten minutes, and it is the most common thing an owner can fix themselves.

The shower screen wears and clogs. Also cheap.

The steam wand tip blocks with milk residue if it is not purged and wiped every time.

A realistic schedule for a daily machine: water backflush after the last shot, wipe and purge the wand every time, detergent backflush weekly, screen and gasket check monthly, descale on a frequency set by your water, gasket replaced annually or when it hardens.

That is a few minutes a day and an hour every so often, and it is the difference between a machine that gets replaced and one that gets repaired.

On your immediate situation: do the backflush and the screen this weekend, get your water hardness figure, and plan the descale for next weekend once you know what you are dealing with. Two years is not too late, it is just overdue.