Update on M-disc:
The drive & discs showed up in the mail today. I got right to work on it.
I just burned a 100Gb M-Disc (BDXL) with a typical drive, nothing costly or special, using free software (ImgBurn) and it detected the full capacity (100Gb) and layers of the BDXL without any effort or special settings and the drive wrote the files to the 100Gb disc without any error, followed by a complete verification post-write at low speed (4x) to ensure accuracy of the data. The drive is a LG WH16NS40 from May of 2020 per its engraving of manufacture off Amazon ($65 new), latest firmware (nothing special for this though). The disc is a Verbatim M-Disc BD-R branded surface BDXL 100Gb multi-layer disc ($11 and change, but I bought a couple in a pack).
For this disc content, I did our 2013's photos. Just the RAWs and JPGs and that's it. It was 95Gb (after culling chimps out) so it fit just right on the 100Gb disc. I did not write on the disc with a market, and will not, to avoid any chemical reaction over time. File system is UDF, and it handled deep directories (8 directory trees) and long file names and folder names without issue. This is read natively by any OS on the planet. I checked with a Windows and Linux platform just to satisfy my own curiosity.
This was successful and error free, so this backup copy of the photos will now go into a fireproof/waterproof safe and be the 3rd physical copy of the data (the other two physical copies life on redundant drives that are separated), it will be hearty to environmental conditions and not suffer magnetic issues, or dye/pigment issues, and no mechanical failure options. Should last my lifetime and then some in fairly well controlled settings. $11 and change for the disc covers all our family photos for that year. I'll do each year separately until its all archived. Then it all goes into the safe.
I will test the disc in a separate drive, likely an external, to see how it is handled; since these drives are so low cost, having one or two new ones in packing to read the backups and refreshing hardware every decade or so should handle file retrieval if needed.
So far this is about the cheapest true backup that will survive most issues and be handled with the most common and least expensive hardware with the least complexity that I can figure without doing the cloud approach.
The capacity per cost is not the best compared to other mediums, but the survivability of the medium was the goal. As a 3rd physical copy that is. Not all of my data will be treated this way. But things like family photos, documents, etc, will all be "engraved into stone" so to speak with this for now. Unless a better method comes up in the future.
Very best,