Hello @Mardi,
indeed many questions – and no simple and complete answers.
... but is it true that I should install OpenWrt via the serial connector?
Most important remark first: If you want to install OpenWrt on a Acer Connect Vero W6m, you have to (not “should”) use the serial interface, and you have to use an appropriate (e.g. pogo pin) adapter or have to solder a three pin connector because there are only through-holes and no connector present on the W6/W6d/W6m board.
... can I use the software for the Predator W6 from the Table of Hardware, since it's the only officially supported version, but will everything work correctly - correct? Mesh, all 3 bands (2,4 5 and 6GHz ), MIMU, Beamforming, Fast roaming, etc?
You can use the W6 snapshot sysupgrade image on the W6m, but there is a brand new snapshot sysupgrade image customized for the W6m you should prefer. As far as I know, everything you mentioned works correctly using the snapshots, the LEDs still don't. (But you can control them via i2c-tools if you know what you are doing.) You should not use the stable OpenWrt versions up to 23.05.5 because of one major bug.
And avoid automatic updates of the Acer firmware in case there is an update that 'locks us out'?
That has already been discussed without clear answer.
I am a bit afraid of bricking the unit, and I like the possibility of restoring it to factory settings and firmware
There is a real chance of bricking the Acer W6/W6d/W6m without any possibility to recover the device. You can generate backups of all /dev/mmcblk0 partitions (i.e. p5 and p6 for Acer's OpenWrt firmware in slot 0, and p7 and p8 in slot 1) before overwriting any of these partitions during the first sysupgrade, but I do not know it anybody has ever tried to restore Acer's stock firmware. (And how could that be done after “something happened with OpenWrt”?)
the W6m seems like a much better option - are there any apparent downsides that I am not seeing?
I think you have noticed all the disadvantages of the W6/W6d/W6m already. All in all these devices are powerful, but complicated to upgrade with OpenWrt and easy to brick. And sysupgrading OpenWrt to the latest snapshot version frequently is really no fun, especially for OpenWrt beginners (as I still am btw).