Considering going with a fresh start on all my devices. Trying a new way of working with my databases and using more markdown files rather than hoarding pdfs.
I don’t want to completely get rid of the current database, but I’d like to remove it from active use. (Just in case there might be a file I need later.) Would archiving the database be a good method for preserving and removing it?
I have had this situation: I can’t assume it is indentical to yours. Archiving works well however I would be inclined just to keep the databases closed for a while at least. Especially if they are not very large. I have databases I haven’t opened for years and there is no strain on my workflow, storage, back ups or anything really.
How does closing a database work with DTTG?
You can’t close databases in DEVONthink To Go.
Archiving the database is an individual decision.
You also can create a new database and keep the old one open while you start building the new one. Then after you’ve worked with it for awhile – and not just a few days –you can assess which works better for you. If it’s the new one, you can archive and remove the old one.
What I do is just not link to the relevant database. Is that not recommended? I just keep it off the sync in other words. Frankly my set up is now so old I have forgotten how I do it. I hardly use DTTG, for various reasons, nowadays.
I only ever had two databases synched to DTTG, the Global Inbox mostly and another one that is a sort of ‘docs in play’ one.
That’s fine if it’s suits your purposes, especially for a database you haven’t used forever. My suggestion is based on iterating a new database. Having the old one open allows you to: access any necessary items you may not have transitioned to the new one, but also continue to use the old one, as the situation requires.
As an example, I do most of my work in a DEVONthink database and have for almost 11 years now. I’m not still using my first database. 
I actually just started the 4th revision, so I have r3 open but am working mostly in r4.
That is pretty much what I do when I make a new database and the pattern seems very similar to yours!