Enable the editing toolbar) This should be already enabled, if you mean "Enable enhanced editing toolbar". If you mean something else, please have a look at https://www.mediawiki.org/wiki/Manual:$wgDefaultUserOptions and let me know which one you wish to get enabled by default.
Enable the visual editor and the new wikitext mode in Structured Discussions) Should be already enabled by default.
Hide categorization of pages) Should be already enabled by default.
Show the pending changes diff when viewing the latest pending revision) I can't find that in my preferences on arwiki.
Add pages I review to my watchlist) I can't find that in my preferences on arwiki.
Hide categorization of pages) Already true by default.
Add new files I upload to my watchlist) Already true by default.
Beta features) I'm unsure how this works internally. Anyone knows if this can be controled by wgDefaultUserOptions?

Change 501926 had a related patch set uploaded (by Urbanecm; owner: Urbanecm):
[operations/mediawiki-config@master] Change arwiki's default user preferences

https://gerrit.wikimedia.org/r/501926

@Urbanecm It'll be enabled for new users only? Or for all registered users?

For all users who haven't changed their preferences. It isn't possible to change a preference for new users only, since the table which stores user's preferences only stores changrs from the default state.

In T220186#5090457, @Urbanecm wrote:

Enable the editing toolbar) This should be already enabled,

Looks good,

Show the pending changes diff when viewing the latest pending revision) I can't find that in my preferences on arwiki.

That would be:
عرض مقارنة التغييرات بالانتظار عند عرض أحدث مراجعة بالانتظار

MediaWiki:Flaggedrevs-prefs-viewdiffs

Add pages I review to my watchlist) I can't find that in my preferences on arwiki.

أضف الصفحات التي أراجعها إلى قائمة مراقبتي

MediaWiki:Flaggedrevs-prefs-watch

Thanks. I'm sorry for the delay, I need to figure out the codename for the preferences. Will try to have a look at it next week.

This is all ready, except beta section. Per looking into arwiki's user_properties database table, it looks it should be possible to auto-enable a beta feature using the same approach. However, beta features are in beta for a reason, and I'm not sure if processing wouldn't cause anything unwanted. Unless someone more experienced confirms this is possible, I'm not going to do that.

Change 501926 merged by jenkins-bot:
[operations/mediawiki-config@master] Change arwiki's default user preferences

https://gerrit.wikimedia.org/r/501926

Mentioned in SAL (#wikimedia-operations) [2019-05-29T16:59:34Z] <urbanecm@deploy1001> Synchronized wmf-config/InitialiseSettings.php: [[:gerrit:501926|Change arwiki default user preferences]], part 1/3 (T220186) (duration: 00m 56s)

Mentioned in SAL (#wikimedia-operations) [2019-05-29T17:00:45Z] <urbanecm@deploy1001> Synchronized wmf-config/flaggedrevs.php: [[:gerrit:501926|Change arwiki default user preferences]], part 2/3 (T220186) (duration: 00m 56s)

Mentioned in SAL (#wikimedia-operations) [2019-05-29T17:02:03Z] <urbanecm@deploy1001> Synchronized wmf-config/CommonSettings.php: [[:gerrit:501926|Change arwiki default user preferences]], part 3/3 (T220186) (duration: 00m 56s)