Please HELP with cycle issues!

Hello. I'm new here. About two months ago I stupidly had a few too many beers and won two goldfish at a fair. Having no idea what to do with them, but being someone who doesn't want animals to suffer on my watch, I started doing my research and realized I should have been getting a tank ready for them months before bringing them home.

I went out and bought them a 29 gallon and started cycling with Dr. Tim's right away. For the wait, I purchased a 5 gallon with an integrated filtration system for their temporary home. Despite obsessive water testing (multiple times daily. I was monitoring VERY carefully), treating, and daily partial water changes, the fish got sick and died after just about a month. It was just as well, I suppose, because their tank wasn't fully cycled yet when I left for a 2 week vacation soon after they passed. However, I had so much fun playing with tanks that I decided I'm going to keep them going and get a betta for the 5 gallon and some guppies for the 29 gallon.

So, here's the deal: The tank wasn't FULLY cycled when I left for vacation, but it was really close. This was almost a month ago now. The ammonia-eating bacteria were fully established and took care of the ammonia I fed the tank in under a day. I never got a "0" Nitrite reading, but it got close after 2-3 days from when I added ammonia. And Nitrates were high before I did 50% water changes every week. So that's where I was when I left on my vacation.

When I left, I kept the tank at about 75 degrees and kept the water filter going. I had a few catappa leaves in there because we have hard water with about 8.2 ph that is very hard to lower. I got home, tested the water, and I had 0 ammonia, 0 nitrite, and really high nitrate (20-40ppm).

I hadn't really decorated the tank before because I was just trying to get it going as fast as possible, so I fully removed the water without disturbing the substrate too much (just aquarium gravel), added in some decorations, and filled it back up. Decorations included fake plants, some fake rock displays, and a few marimo moss balls. Plus three pieces of cholla wood (again to try to control PH a little). The cholla wood was a fail and it came out of the tank after five days because it won't sink, despite me boiling it for over an hour before putting it in.

The problem is that now my tank seems like it's not cycled. After I added the decor and filled the tank back up, I dosed with ammonia to see how fast that would cycle through (2 drops per gallon). It's three days later now and the ammonia reading hasn't moved (about .50 ppm). There is 0 nitrite, but the Nitrate is high again at 10-20 ppm.

Can anybody help me with what may be going on? I didn't think that I did anything that would have killed the bacteria in my tank while I was gone. Could it be the new tank decorations? Specifically the cholla wood or marimo balls? I'm so confused about why this all just collapsed so quickly. Thanks for any thoughts or advice!