Don’t use the Switch’s recording system, you’re right that it only records 30s so it doesn’t work for recording speedruns. Either invest in a capture card or use a camera/phone to record the screen some other way. Anything is fine as long as the gameplay is clear, but a recording isn’t required unless you have a top time. It is nice to see, though, if you have the means to do so.

I could help: I'd recommend the Avermedia LGP2. It's easy to set up, has decent video quality (up to 1080p60fps) and has a good price for what it can do. You can hook it to a PC ir just use it with an SD card (minimum has to be Type 10 with 16GB of storage space). The Plus versión is a little more expensive but can record in 4K60fps (not that you need that on Switch).

Hopefully this helps!

I use an Elgato HD60. You just take the HDMI from the Switch, plug it into the capture card, plug an HDMI from the capture card into your TV, then plug the USB from the capture card into a computer. There’s recording software that you download from their website, but I use a video input capture source in OBS. There are probably tutorials to help you out on YouTube.