Have you thought about cutting out the bottom panel and replacing with another sheet of 1/2"? That would fix all of your holes and could even allow you to move the weir location with a little more work.
I've removed a top panel with a router and welded on a new panel in about an hour. Much faster and easier than building a plywood aquarium, but doesn't change the footprint of the tank. If you're going wider or longer on the new build, disregard my last comment.
I considered it and it is still an option although I don't have a ton of experience welding acrylic. There really are some things I need to figure out before I do anything else at this point.
1. Do I want to stay in the hobby? I'm just not enjoying my current aquarium and I'm not sure if it is because of the fish I am keeping or if I am not into keeping fish like I have been most of my life.
If the answer is no, I need to find suitable homes for my current fish and then dismantle and get rid of everything.
If yes, what does staying in the hobby look like for me? Do I go big, do I go small, do I stick with what I have with different fish?
2. If I do want to stay in the hobby, would I be happier with a single specimen tank that can live in my current aquarium for life, such as Fahaka Puffer, or some kind of medium sized guapote?
I kept a Fahaka for years in the past. It was probably my favorite fish I have ever kept. I could see myself going back to something like that. Or maybe a single show specimen of a guapote like a Haitensis, Managuense, Motaguense, etc...
3. If I decide I want to stay in the hobby, what do I want to keep long term and what aquarium dimensions will I need for that?
If I decide to put in the time, effort and money into a larger aquarium what am I going to keep in it? I live in Arizona so freshwater stingrays are off the list. My ideal aquarium would be 360 gallons with the following dimensions 8 feet long, 3 feet wide, 2 feet tall. So if I want a larger aquarium, I would likely harvest an 8 foot sheet from the 300 gallon and build a 360 like I have always wanted and use my current acrylic aquarium as the sump.
So before I do anything else I need to do some soul searching. I'm ok with waiting and taking my time. I also have career circumstances that may decide this all for me. Selling my house and moving into an apartment. International travel for extended periods of time, etc...