Hi Oli1000.

Welcome to the RME user forum.

> I need a least 10 extra inputs but I don't know what would be the best choice.

Assume you mean analog inputs. You can only route 8ch through ADAT at single speed.

Option 1: limits you to single speed, and you can route only 8 channels through ADAT. For 2 more channels you could get an SPDIF to AES cable and connect through coax SPDIF.

Option 2: is possible. UFX II is fully manageable in stand-alone mode.

Option 3: a 2nd UFX II for routing only 8ch over ADAT is expensive. 2 more channels through AES.

Option 4: correct, 2ch are missing, get an ADI-2 Pro and connect through AES Nice improvement for monitoring section.

Option 5: gives you most flexibility
                UFX+: 4+8=12 channels (44.1 - 192 kHz)
                Through MADI XTC, M-1610 Pro: 16+8=24 ch (44.1 - 96 kHz)

Option 6: Get, e.g. from used market: UFX+, ADI-2 Pro FS, XTC #2 and sell UFX II.
                The two XTC's are remote controllable:
                a) through TotalMix FX (Auxdevice) settings can be stored / recalled through TM FX snapshots
                b) through MIDI Remote AVB

UFX+-+<---------------------------------------------------- 4x analog I/O IN
|   |  \ +<---------------------------------------------------- 8x analog I/O IN
|   |   \+<-AES----ADI-2 Pro [FS[R BE]]<---1/2-------- 2x analog I/O IN
|   |                                                     +---1/2------> monitoring: active monitors
|   | MADI                                            +---3/4------> monitoring: phones
|   |
|   +----------------Octamic XTC #1<--------------------- 8x analog I/O IN
|   +----------------Octamic XTC #2<--------------------- 8x analog I/O IN
+--+

Option 7: Get: UFX+, M-1610 Pro, sell UFX II.
                I left out the ADI-2 Pro here, of course it would also be great here in the monitoring section.

UFX+-+<---------------------------------------------------- 4x analog I/O IN
|   |  \ +<---------------------------------------------------- 8x analog I/O IN
|   |
|   | MADI
|   |
|   +----------------Octamic XTC <------------------------  8x analog I/O IN
|   +----------------M-1610 Pro  <------------------------ 16x analog I/O IN
+--+

I would keep the XTC because it gives you a lot of flexibility as a format converter.
And also ADAT and 4x AES outputs, this gives you a great flexibility.

A digiface USB would give you 32 inputs at 44 or 48k on adat ports. Just saying.
And just a thought, but if you don't need to record all 30 synths at the same time, would a (maybe digital) mixer not be ideal?
I have about 8 devices (16 channels stereo) coneected like you want and keeping track of everything happening in totalmix is not always that much fun. So often, why don't I hear my synth, oh I muted it last week... Haveing just a physical fader per synth would be nice (but alas no budget)

Loss of features, DURec, etc.

Depends on how much space you have and how many mixers you want to deploy for so many synths.
An UFX based solution can be racked and is more compact and TM FX operation is extremely comfortably and reliable
if you know how to use it.

You could use “Layout Presets” in TM FX  to organize the display of according to the devices, in his case with UFX+ and two XTCs:
  - Layout Preset 1: Name: “UFX Ana”  : analog inputs from UFX+
  - Layout Preset 2: Name: “XTC1 Ana”: analog inputs from UFX+
  - Layout Preset 3: Name: “XTC2 Ana”: analog inputs from UFX+
  - Layout Preset 4: Name: “ALL”         : all HW input channels from all devices

Furthermore, you can make good use in this case of Workspace Quick Select slots.
By this, you can quickly recall a workspace quick select settings by using the key combination ALT-1 - ALT0 (you have 10 (!) different workplaces with 8 snapshots / settings directly at your fingertips).

So you could clone a basic TM FX setting with the Layout presets and then unmute different groups of channels.
Maybe again per device.
And then save this into different Workspace Quick Select slots.
Then you can use ALT-1 to e.g. unmute all inputs of the UFX+, ALT-2 to unmute all analog inputs from XTC1.

Good HW mixer are not inexpensive and need a lot of space.

This is only an example, maybe you find a better combination for your personal demand.

You can even make use of the ARC USB and mute / unmute groups of ports in 8 different snapshots per workplace.
Also in “Quick Select Workplaces”.

And regarding using fewer ports … Depends … might be very uncomfortably, esp if you have special racks to place many synthesizers ready for use to support the creative process.