It seems to be a daily antacid for your horse? Is it intended to be used long term or as a follow up with omeprazole to step your horse down? 

What maintenance supplements are recommended? I've looked at GastroPlex, U-Gard, Uckele GUT, SmartGut, etc. 

U-Gard can be used daily, probably best as a daily. For horses that have had ulcers, they will most likely keep coming back. When giving omeprazole, always start a daily preventative at the same time. Omeprazole can cause ulcers once it is stopped, so they need to be on a daily preventer thats had time in their system to take effect, usually 30 days. Best to continue a prevention daily long term. Pretty much any ulcer preventer, like you listed is meant to be given long term. So you're fine there. I personally use Gastro-plex on mine as it covers hind gut as well, which others don't. It also reduces colic risk, so it's super nice peace of mind when horses are colicy during those weird weather swings, feed changes, etc. 

I have used the U Gard powder with good results but not the pellets. My horse seems to do better on ulcer supplements that coat the stomach (powder or liquid).

I have also had good results using U-Shield and Gastro Elm.  

Because of the Calcium Carbonate and Magnesium in U-Gard, it looks like it would be a good buffer, but I don't really see any ingredients that are going to help promote a healthy gut and digestive function. It looks to be comparable to a human taking Tums - which we know provides comfort but doesn't actually address the root of the problem. 

Gastro-Plex contains ingredients (antioxidants, probiotics/prebiotics, herbals) that support the stomach, protect it from the abbrasiveness of stomach acid, and also contains Levucell SC, a yeast proven to reach and support the hind gut. 

I've had good success with Jeremiah's Ulcer Repulser.  https://www.ulcerrepulser.com/ I mix it with Aloe Juice from Walmart and they lick the feeder clean.  Check out their website.  It costs $50.00 a month which is a huge savings comparably!