radmon.org outage 28/3 - 01/06

The concept is simple enough, but the practice seems to be harder to pin down.  I know Dan mentioned wear levelling when setting it up, so I'm sure settings were made to that end, but the RPi and Rasbian with its 'roll your own' basis means there is generally less attention paid by driver writers to make this easy.  When I got a laptop circa 2002 with an SSD, I did not expect it to last 19+ years under almost daily use.  I recall a setup question to the effect of "Speed, size or reliability, pick 1 or 2 only".  Now I'm glad I picked only reliability.  The RPi driver or set of drivers likely can be tuned in a similar way, but may be less proven and may provide no feedback over time.  The few times I have worked with one-off RPi's, I found myself spending 100% effort getting it to work and 0% in optimization.

About the SSDs being unfit for many transactions (reads/writes), that's no longer the case.
My server (DL380 G9) runs on full flash and serves over 10 guests, DBs, mail and websites. The SSDs (8x 1.6 Intel DC versions) are in RAID 10 and have a wear&tear of 1-5% for 2 years production date.
Then my NAS (also a DL380G9) uses two SSDs for booting/ running the OS and then 2x NVME for caching.

In the company I work for as a sysadmin we have a full flash HCI cluster running over 100 guests for over 3 years and no SSD has died over that time. Wear level is also low.
This cluster hosts a multiple of guests, DBs, fileserver, our own software, etc... So in short many read,writes and IOPS

The lower the cost of the SSD -> the lower the TBW it can handle, on ebay you can source Enterprise SSD (Intel DC versions, ...) for cheap, which will give you a higher then normal TBW in regard to NAS/ home usage disks.
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@mw0uzo, in case the endurance flash card would fail, perhaps an USB to SATA convertor could help with a "cheap" Intel DC SSD or enterprise HDD, I also believe that they make enclosures for PI that can incorporate it directly with active cooling, so you have a small bootprint enclosure.
Be carefull with the heat dissapation, SSDs do not really like high temps (above +50C). If the PIs have reached their limit maybe a small form factor computer could help?

e.g. https://www.bargainhardware.co.uk/lenovo-thinkcentre-m900-tiny-pre-configured-desktop-pc
Then you could mix SSD/NVME/HDD while keeping the cost low and stability/ endurance high.

My two cents

That sounds about right, we're running enterprise SAS SSD's in RAIDs & seeing 1% wear/year. But they also very much don't fit the use case of a solar powered Raspberry Pi.

Something I've tried that does fit the use case better than thumbdrives is NVMe to USB 3.0 (and prior to that, msata-to-USB 3.0 but I've not compared power usage) boards. They typically have better management code, a larger array, and the adapter/controller board itself leaves the VRM exposed so if needed a heatsink can be fitted. Ideally, a solution will be available that doesn't require that conversion waste. Also, NVMe's can be fitted with a heatsink to both maintain performance and increase longevity.

[Update 2021-06-17] I just did some quick research and while some NVMe's are as low as 0.125 watts at idle & the situation is fluid, in practice and taking OS power management into account...they can be terrible power wasters at idle, often using tens of watts. mSATA typically uses around 0.3 watts at idle vs 0.8 or worse for a spinning platter.

Regarding Pi cases with active cooling...there are also cases where the entire case is a passive heatsink. I've been using them on a few projects and even in a somewhat hostile environment they keep the temps low. One thing I would recommend though is be sure to get one where each half is cast in a single piece, not the case + a spacer to make contact with each chip; that's 2x the interfaces and it lowers efficiency.
Speaking of which, the thermal tape/pad they usually provide is um...sub-optimal & buying a small separate sheet of higher quality is worth it.

Example of integrated, 1-piece thermal contact with chips: https://images-na.ssl-images-amazon.com/images/I/61XugLVhJQL._AC_SL1000_.jpg

Also, on (R-Pi 3 in one particular case I had) cases that did not make connection on the bottom, a sheet of thermal pad often worked wonders if the gap was small.

Finally, a separate thermally controlled small fan can be mounted to the outside of the passive cases with sheet metal screws similar to how old Socket-7 heatsinks were made. I've been planning to add that to one of mine that needs to be up in the attic for RF reception.