3.) What is your backup strategy?

I'll start with that first since although that's the EASIEST question to answer no one ever seems to actually like the answer and often don't follow the advice. Then later they wish they had.

BACKUP - BACKUP - BACKUP

1.) What is your strategy to test and check new sounds / libaries without creating a mess in your synthesizer ?

Hmmm - did I mention that you should ALWAYS have everything backed up? IF you do then it is simply NOT POSSIBLE to have a 'mess' in your synthesizer since you can always start clean.

BEFORE starting any new testing/checking make sure you have the current state of your system completely backed up. That means a new backup file, user file and copies of things like MIDI/WAV files that you need as well as your arpeggios and library files.

That allows you to restore the system to the starting point no matter how much you may have changed things during your testing.

2.) How do you get an overview over 10000 Arpeggios?

Unfortunately there isn't any way to do that. You can get some hints from the info in the Arpeggio section of the Data List doc and some hints from the abbreviations used in the names of the arpeggios.

But other than that you have to just try them yourself. There isn't even any way to know which preset performances use any particular arpeggios.

4.) How do you prepare your life setup?

Ideally you should have WRITTEN specs for how your system needs to be setup for each of your gigs. Those 'specs' would include the name of the USER backup file, library files and other files needed for each gig.

Create a USER backup file that has the setup needed for each gig. Then before each gig restore the appropriate USER backup, library and other files needed for that gig.

Again - creating a proper set of backups makes the setup for a gig much easier and prevents deleting needed stuff.

Without proper backups there can be complications around user waveforms (do you use any?), MIDI setups (zone, channel, controller config) when using the Montage as a controller or when it is being controlled (some parts being played via another keyboard).

There can also some overall utility settings you can make that are NOT saved automatically when you power down but ARE saved in a user file when you create one.

Bad Mister briefly discusses that issue in this reply
https://www.yamahasynth.com/forum/note-shift-vs-transpose#reply-121122

1.) What is your strategy to test and check new sounds / libaries without creating a mess in your synthesizer ?

Montage has 8 Library slots. When you load content in a library (most 3rd party content is offered as a library file) you cannot edit any of the content including deleting unused Performances.

So just to check out a library, load it into an empty slot and this will not impact anything already on your keyboard except possibly consume waveform (i.e. samples) memory of which you have 1.75GB total. Libraries should be published with a size in terms of waveform memory needed.

After checking out the library you can either delete the library and move on or you can leave it alone for later combining with other Libraries.

What you would ultimately do is import the library to your user area. You are able to import only selected Performances from the Library which cuts down the clutter. The function is called library input under [UTILITY] and the "Content" menu. Importing does not disturb anything already in your User area and is a merge operation. Therefore content from multiple Libraries can be combined.

2.) How do you get an overview over 10000 Arpeggios?

Not easily. What you have a available to you are categories and subcategories which is how you get in the ballpark then you can select different arpeggios from the arp search and listen to what it does.

3.) What is your backup strategy?

Whenever I do anything that's a possible time when I could lose data I make a backup to an "all" file which will save everything. This includes before firmware updates or if I'm going to start touching the user area in any significant way including a major Library import. Also it's good to backup after adding a lot of user content.

4.) How do you prepare your life setup?

First I build up all of my custom Performances in the user area. Typically I'll take single Part performances and combine these (Category Search Performance merge) including splits and other customizations. I store my new Performances with recognizable names that are easy to search for.

Then I add these Performances to Live Set. On the gig I recall everything from Live Set which gives a 4x4 grid of Performances ordered however I like. There's an extra field where I may add performance notes to help me use the sound. It's easy to change the order of Live Set if you want. I just keep everything in a fixed place. Most of my gig on on the first page of 4x4 Performances. At most I would have to page down for the next page of 16 Live Set Performances. My keyboard boots up into Live Set mode so on a gig I never leave this screen.