Hi,

I have a planted tank of 160L/about 40 US gallons. I had a few problems and want to get a center piece fish, but I also want to ask what might've gone wrong with my betta's. So this is a long post, sorry. I'll put a tl;dr in the end.

Currently there are 6 celebes rainbowfish, 2 hillstream loaches (P. Myersi), a whole bunch of bronze corydoras (they had offspring, ~12 fish), and 4 amano xl shrimps.

No ammonia and nitrites, I did get way too high nitrates but that's more and more under control (~50ppm now). The filter is a Sicce Whale 350 and there are two bubblers (Eheim 200 split in two bubblers IIRC). Temperature is about 25-26°C (77-79F).

I used to keep a singular betta in the tank with the cories and celebes, but I have had the worst luck with them. Three died in about 1.5 years. One disappeared while I was on vacation, so I don't fully know what happened (family came to check up on them, but they were also perplexed). The second one was sickly from the start and kind of wasted away with clear dropsy at the end.
The third one got stuck in a little glass vase I put on the glass with an anubia pinto plant. I got him out but he was barely alive, and although he seemed to slowly get better he eventually died of dropsy (but without clear signs of infection) in the end too.

My current working theory is that the tank is too busy, but as long as the betta is healthy it's fine. Yet betta's can't be fine all the time, and my nitrate spikes didn't help of course.

I also don't give the bigger betta pellets as the main feed anymore since the last one died of dropsy without clear signs of infection (I still have a betta in another 70l/20 gallon planted tank with shrimps), but I'm not fully sure if that's a factor that really mattered. Still, I feel like they can disrupt the digestion (especially since I tend to give too much of it) (I try not to).

I also tend to go for calmer betta's in the LFS that will probably not mess too much with my other fish in the community tank, but the second one was in retrospect too calm and hidey.
In theory they like almost still water, but I have a lot of resting places (esp near the surface), choose betta's with smaller fins and keep an eye out that they don't get thrown around in the tank.

So I'm looking for a centre piece fish with lots of personality, but who will most likely leave my other fish alone. I can return them to the LFS if it doesn't really work out, or if they don't need much space I have a 30l quarantine tank running in the background. So I can manouvre if it doesn't work out, but still.

I tried honey gourami's, but my Celebes fish were scared of them and only stayed between the plants. They were also quite dominant actually. I'm pretty sensitive about agression in my tank.

I'd love to just get another betta, but I don't want to guinea pig one after another till I perhaps find out my tank truly is too busy for them.

Tl;dr: want another fish in stead of betta's, that can handle other busybodies but doesn't dominate. No honey gourami's or livebearers please.