I have a lot of experience with spice but have only smoked with harmalas twice. The first time it was tincture and I don't know how strong it was. I finally got pure harmala salts. Yesterday I tried 180mg and took a couple GVG hits.

It definitely potentiated and prolonged the trip, but not by more than roughly 20 minutes. I've heard of people taking harmalas, smoking and tripping for two hours.

I assume I had a low level of MAOI inibition at 180mg. I would really like to extend the dmt peak as long as possible. However some of the side effects of full inhibition don't sound too pleasant.

I want to get as much as possible out of the spice. If 180mg harmala salt gave me a taste of partial inhibition, what would be a good next step? How much should I increase by for a substantially more potentiated experience without too much side effects?

To potentiate the spice, you don't have to use the harmalas orally. Since your vaping, you could smoke a 20-30mg dose of harmalas, or take it sublingually, and this will potentiate vaped spice. I have done this before and achieved a trip that last for over an hour.

The side effects and nausea from the harmalas are not as noticeable with these ingestion methods, imo. It just won't potentiate stuff you take orally.

Make changa. I was having trouble getting smoked harmalas to stretch the dmt effects past 20-30minutes smoked separate but with changa it doesn't seem to be a problem.
Rue harmalas are more potent than caapi harmalas but a little harsher on the side effects ime.

Try changa with different ratios of harmalas & figure out what works best with your body chemistry.
1:1:0.3 herbs/dmt/harmalas works nicely for me but some require more harmalas. 1:1:0.5 is heavy & 1:1:1 is extremely potent & i cannot smoke it without getting harmala sickness midway into a nice session.