Q: iPhoto vs Photos

I’m talking about keywords, tagging, search, syncing  and reliable backup.

iPhoto 9.6.1 can do that, but it cannot use iCloud Photo Library. And iPhoto is a dead end. Apple is no longer selling it and stopped already the support for print products.  If you want to start new, use the currently supported application Photos for Mac, version 1.5.  It can upload to iCloud Photo Library and sync your Photos Library across all devices.

2. To save my MacBookAir internal flash storage, could I use my 1TB time capsule to store my photo library and iTunes music library?

Photos cannot work with a library that is on a drive used by Time Machine. Use a locally mounted external drive. any drive yu use with Photos needs to be formatted OS X, Extended (Journaled). And it must not be a drive that is syncing - no DropBox, Google Drive, iCloud Drive.

If you are planning to use iCloud Photo Library, you can enable "Optimize Mac storage" to save storage on your internal drive.  Then Photos will only save smaller versions of your photos locay and retrieve the ful size origina files from iCloud, if you want to work with the photos.

See Apple's document:  Updating from iPhoto to Photos for OS X - Apple Support

If you store your Photos library in a location that's part of a cloud service (for example, Dropbox, Box, or Google Drive), your library might be damaged by the syncing process. As a result, you could lose photos.

To see your photos on all your devices, you can use iCloud Photo Library. To back up your library, you can use Time Machine.

and reliable backup.

Apple does not offer a backup for your Photos Library. on a Mac.  Cloud backup is only available for the camera roll on mobile devices.

iCloud Photo Library is primarily a syncing service.  It stores your Photos in iCloud and syncs them from there to all devices. See:   iCloud Photo Library FAQ

Because of tis syncing iCloud Photo Library is the primary storage of your photos.  All Edits , deletions, additions will be updated immediately in iCloud and propagate across all devices.  If you make a user error and delete a photo or accidentally write wrong metadata to all photos you cannot recover the previous versions fro iCloud Photo Library.  But if your Mac gets stolen or damaged, you can recover the library from iCloud. Ony it is not an archival backup that will protect you from user errors.

So keep a local copy of your Photos Library on an external drive where Time MAchine keep a backup of your library.

. BUT, how to manage and organise 30 thousands pictures without keywords, tagging, and powerful search engine? or should i forget all of this and go with google photos, Flicker or even dropbox?

Photos for Mac lets you assign keywords, titles, captions, locations, faces names. You can create albums and smart albums.

If your photos are currently in an iPhoto Library or Aperture ibrary, the metadata and albums will be migrated to Photos.

See this link: How Photos handles content and metadata from iPhoto and Aperture - Apple Support

It is only Photos on an iPhone or iPad that does not let you assign metadata to the photos.  But if you assign keywords, titles, etc on a mac and sync the photos to the iPhone or iPad using iCloud Photo Library, you can search for them and find the photos.

how to manage and organise 30 thousands pictures without keywords, tagging, and powerful search engine? or should i forget all of this and go with google photos, Flicker or even dropbox?

Have you used iPhoto in the past and assigned keyword, titles, etc. to your photo?  If so, when you migrate your iPhoto library to Photos all of those will be included in the new Photos library.

If you haven't used iPhoto previously just select photos you want to batch assign a keyword to and use the key combination of Command + K to bring up the Keyword pane.  Click on any keyword you want to assign to all selected photos.  That keyword will then be shown in blue.

As for other tagging you can batch change the title (what's shown under the thumbnail images, and description by using the Info pane (Command + i) or using one of the Applescripts in the Photos for Mac  User Tips section.

Searching is very powerful using Smart albums.  You can search on any of these items or combination thereof: