Used Octamic II still worth it?

A 12Mic is not in the budget ? Then the Octamic is a good choice.
The price is a bit high but depending on the condition it is ok.
Was the unit always in smoke-free room ?

It has knobs for manually gain adjustment, no digital control, this is the downside of this device compared to the 12Mic.
It has phantom power on each input ON/OFF with soft-start, voltage lift up during 1 to 2 seconds or so, no popping noise and the microphones will thank you.

The sound quality is fine, clean and neutral.
It does exactly what the job is.

It is reliable, 10 years is not old.

This is difficult to answer as you might have budget restrictions.

From the used market you could also consider buying an Octamic XTC. It also gives you MADI as an option.
It can also be integrated as Auxdevice into TotalMix FX, so that you can remote control the preamp using TotalMix FX.
Then you can store / recall all relevant preamp settings using TotalMix FX.

Auxdevice works with MIDI and if you have a MADI capable setup, also versus MIDI over MADI over this one fiber cable.
Alternatively you can also remote control the devices via RME connector (also via MIDI or MIDI over MADI).
If you had a 12Mic then via LAN is also possible when using RME connector.

I personally am a big fan of having all under the hood of TotalMix FX.
So an XTC with Auxdevice support (either over MIDI or MIDI over MADI) is IMHO the best thing and really desireable).

I would say this is less a matter of converters but features, like described above.
Other things to consider:
- supported preamp gain (12Mic has up to 75 dB like the UCX, UFX II/III), interesting if you need it)
- max input level (XTC has PAD for 4 ports)
- other features like AES sync port or not, ADAT IN or not, Autoset or not, MADI or not
- whether you like to have the ports in the front or in the back

Maybe the best option is, of course, the 12Mic. Higher port density and support besides MADI also AVB or Dante.
On the other hand, especially the XTC is an excellent product, focusing on being a MADI (only) device.

It supports even 4x AES I/O and one ADAT input so that you can use it as a format converter between AES, ADAT and MADI.
Another very uniq feature of the XTC is that it supports Class Control and can work as a USB interface, e.g., for the iPAD.

If I were to get a used device and if I had to look to the price as well, then I would decide for the "middle".
I think I would focus on the XTC which has the richest feature set of all and supports even (exclusively) MADI.
If you do not intend to use AVB or Dante (which is a more complex and pricier setup), then an XTC from used market is surely the sweet spot for you.