Third corrupted card in a year - is booting from an SSD the fix, or am I doing something else wrong?

Two separate causes and it is worth knowing which you have, because an SSD fixes one of them properly and only partly helps with the other.

Wear from writes. Flash cells have a limited number of write cycles, and cards use much simpler wear levelling than an SSD. A machine writing continuously - logs, a database, metrics, a cache: chews through that far faster than people expect. Four months of constant logging is entirely plausible.

Corruption from unclean power loss. A card being written to when power disappears can be left inconsistent. This is often the real cause on boards that get unplugged, or that brown out under load with an inadequate supply.

How to tell: if your services write a lot, suspect wear. If your corruptions follow power events or you have ever seen undervoltage warnings, suspect power - and see the next answer.

Things that reduce writes considerably:

Send logs to memory rather than to the card, and cap their size.

Move any database or frequently written directory off the card, to a USB drive or over the network.

Mount things read-only where you can.

Reduce swap, which on a card is a write amplifier.

Do those and cards last a great deal longer.

Booting from an SSD is the right answer for anything running continuously, and it is not only about failure rate.

What you get:

Far more write endurance and proper wear levelling, so ordinary logging stops being a countdown.

Much better random performance, which is what actually makes these boards feel slow. Databases and package updates in particular are transformed.

Better failure behaviour. SSDs tend to degrade in ways you can see coming through their health reporting, rather than dying silently.

Practical notes:

Check your board can boot from USB or has an NVMe option, and update the firmware first if needed.

Power matters more with an SSD attached. A drive drawing from the board's USB is a real load, and an underpowered setup that was stable before can start browning out. Use a good supply, and a powered enclosure if in doubt.

Not all USB enclosures behave. Some controllers are unreliable on these boards. Check what people report for the specific enclosure before buying.

And keep the card habit that matters regardless: a known-good image and your configuration backed up somewhere else. An SSD makes reflashing rare rather than impossible.

Before you buy anything, rule out power, because it is cheap to check and it is the cause people miss.

These boards are sensitive to supply voltage and many failures blamed on storage are actually the board browning out under load and corrupting whatever it was writing.

What to check:

Look for undervoltage warnings in the system log or the on-screen indicator. Any at all means the supply or the cable is inadequate.

Use the official supply or a genuinely equivalent one. A phone charger with the right connector is not the same thing, and neither is a long thin cable: a lot of the loss is in the cable rather than the adaptor.

Count what is drawing from the board. Drives, cameras and hats all take power from the same budget.

Shut down cleanly. Pulling the plug on a running system is the classic corruption cause, and if the board lives somewhere it gets switched off at the wall, that is your explanation.

If you find undervoltage, fix that first - an SSD on an underpowered board will also misbehave, exactly as you suspected.

If power is clean and the writes are heavy, then it is wear, and the SSD is the correct purchase rather than a workaround.