Not excessive, some people like @seangee do 75% every week, I do between 50 and 75% per week, and larger water changes are better than smaller ones. When it's about ammonia/nitrites, larger ones are safer, especially if it's likely to rise again, like in an uncycled tank. Doing a 75% change instead of a 50% is more likely to reduce it to zero than a 50% change and buy more time until the next days test/water change.
The only times I can think of when a large water change would be bad news would be if the temperature wasn't matched or at least very close, since a sudden big swing in temp will put fish into shock, often fatally - or if the tank has gone without large water changes for a long time.
Like when I took over maintaining my dad's nightmare tank, and learned about the nitrogen cycle and water changes. For years he'd only been using soft rainwater, not doing regular water changes, and mostly just topping up the tank, not changing out water. Over time, this means lots of different aspects of the water chemistry have drifted away from the chemistry of the source water, and of course, nitrates were sky high. Off the API charts kind of high, well over 200ppm. I know it must have been that high since it took several partial water changes just to bring it down to a readable level, let alone to 5-10ppm.
Since the tank water was so different from source water and tap water, and the fish had done their best to adapt to these over the years, if I'd gone straight in with a 75% tap water change, I'd have likely killed the whole tank. Aquarium co op has a good video on 'old tank syndrome' which helped me a lot. I had to do many small water changes over a period of weeks, both to reduce the nitrate level gradually, and the other untestable water chemistry differences, but also to gradually move the tank over to a mix of rainwater and tapwater, gradually increasing the hardness.
So that's one reason we advocate for large weekly water changes. Tank chemistry drifts over time, things like pH and lots of other qualities, so large changes keep your tank water chemically close to your source water in many ways, not just in terms of reducing ammonia/nitrites/nitrates. With old tank syndrome, should an emergency large water change be needed, like if a tank broke, a whole tub of fish food got dumped in etc, the large water change is essential, but just as dangerous to the fish because of the large and sudden change in so many aspects of the water chemistry. Stable levels and gradual changes are even more important to fish than ideal levels. That's one reason we say not to try to alter your pH using chemicals. A constantly fluctuating pH is worse for the fish than a pH that is outside of their ideal range.
Sorry for the essay! All that to say, yes, larger water changes tend to be better than smaller ones, except in a few cases. It's one reason the template asks how often and how large water changes tend to be for someone who is experiencing a problem.
Since you lost some fish before adjusted the temp, can't tell whether the temperature changes or the original cause of the fish deaths is to blame for the baby cardinals. I'm sorry for your losses
But the temp has fluctuated a lot over the last few days, that increases stress, and yes, 30 degrees C (86 F) is too hot for them to be maintained at. It should be survivable for them though, since that's the temp we'd raise to to treat ich for a couple of weeks, but wouldn't be safe in the long term. You do need to reduce it though and try to stablise it, adjust only a degree or two at a time, giving them a few hours to adjust before dropping it again.
Are you able to purchase a liquid test kit like the API one? Dip strips are not the most accurate, and it might be making all of this harder to figure out. API kits are good value since they're not only more accurate, but you get hundreds of tests out of a kit, compared to paying the same price for maybe 50 or so potentially inaccurate dip tests.
That level of nitites though is dangerous, I'd do another 75% water change, then test again, and possibly test your tap water, see if something strange is going on there.
