Is this a sign that lots of people erroneously set their rank as 30 kyu (whether on purpose or by accident) as suggested? Or is this actually a sign that we genuinely have a lot of beginners? Without some more detailed analysis of the data, it’s hard to say. 
These are important issues. Thank you for raising them 
To address some of these issues, I suggest the following:
-
Clear guidance on how to select initial rank (avoids beginners thinking 6d is appropriate)
-
New accounts declare rank (if known) to set initial Glicko rating but rank still shows as [?] until deviation drops below existing threshold (avoids ego chasers, etc.)
-
Option to reject games from [?] accounts (allows existing OGS dans looking for serious games to avoid new accounts erroneously set to Dan and allows established 20-30 kyu accounts some protection from sandbaggers)
This is a really good idea 
Coupled with my point above that even with declared rank it is still high deviation and shows as [?], this could work quite well. It could even be a couple of questions on the registration page. Something like this:
Ranks allow matching of player abilities for fair games. Most new accounts start with a provisional rank that shows as [?]. This should settle on an appropriate value automatically after a few games (5-6 on average). The questions below may help to reduce the number of games necessary to get a confirmed rank.
- Do you know your rank? Yes / no
If yes:
-
Enter your rank here: [rank drop down] [association / server drop down]
-
Enter link to your association / server profile: … (This will allow moderators to confirm your rank and skip the [?] stage completely)
If no:
- Select your level:
- Beginner
- Novice
- Intermediate
- Advanced
- Don’t know
(the above would map to, say, 30 kyu, 20 kyu, 10 kyu, 1 Dan and don’t know reverts to current behaviour, i.e. 6 kyu)
This is similar to @Uberdude’s suggestion 
but without the need for any tsumego 
This also covers some of the sign-up process parts of @Groin’s ideas. I also like your idea of assigned coaches for beginners but I think that would be difficult to implement. You would need volunteers and someone to manage assigning of coaches to beginners. I like it in principle but I can’t see if working in practice 