Ok, so you prefer to trust the volume level on the device level, one possibility.
But there are also other possibilities for that and I understood you that you might need more different levels,
so I proposed to control the volume by snapshots, as then you could define even 3 or 4.
Or additionally in combination with DIM as MC proposed in his posting.

What I am doing is to have defined safe settings for each of the snapshots and to load it into the device when it becomes initialized. Additionally the device rememers its last settings.
If your device has different presets its best to load your safe settings to each of the 6 presets to get no surprises when perhaps recalling a preset.

If your monitors have a volume poti, turn it down. Alternatively solve any remaining level mismatches using attenuators, these are very flexible as you can switch them between -10, -20, -30 dB: https://www.thomann.de/de/jts_ma_123.htm
This is better compared to having to turn the volume down too much in the digital domain.

I am doing it in a different way for the same purpose. I am configuring a safe volume setting in each of the 8 snapshots for each monitor/phones port (AN1/2, Phones 9/10, 11/12 also for phones ports of XTC) and ensure, that the connected interface always become initialized with exactly these settings.
By this I can connect any UFX+ safely to my computer even if it became resetted or if you use another of the 6 presets by accident, where the volume level might be different.

For this I am using these Preferences in TM FX:
Snapshots         -> Do not load    "---"
Device Handling -> [ x ] Always init DSP devices with TotalMix FX settings

But this is not so important anymore in my current setup, since the monitors and phones are now connected to an ADI-2 Pro FS which is connected to the AES port of UFX+:
UFX+ ---AES-------> ADI-2 Pro FS R BE------> Monitors/Phones

So I can safely use 0dB there.

When I had the active monitors connected to the UFX+ analog ports in the past I did it in the following way
- set AN 1/2 OUT of UFX+ to the lowest reference level
- turn down the volume at the Gaithain active monitor
- use the described Preference values above and stored a save initial volume of -30 on each of the outputs

Another ideas might be

1. Add ADI-2 Pro to your setup which can be used not only as high quality AD/DA converter but also as kind of monitor controller. Then you can use 0dB on your outputs safely and control the volume there. Nice but expensive way.

2. Get an ARC USB. You can use it for any recording device which supports TM FX. Then you can use e.g. Snapshot 1-3 for up to three different volume levels and additionally use the volume knob or DIM.

Additional safety guts here:
- 8th snapshot is a complete mute of ALL outputs ... kind of a safety off.
- remapped ADI-2 Pro's SETUP key to "Mute all"