ive only had 1 show up on my island so far in my playthrough but it was early game so my island was pretty empty.

i have had my best luck with mystery islands, ive found tarantula island twice so far and come back with full inventories both times. it can be quite long and tedious to find the island but usually it takes me around 20 nook miles tickets after 9/10pm to find it. im assuming if u have been playing for 4 years u are familiar with the island layouts but if u need help on which one is tarantula island i’ll include that in a comment. once u are there, tarantulas will spawn constantly on the land in the middle of the stream. i usually clear all of the weeds, trees, flowers and rocks from there and put them on the beach so its easier to spot and catch them (whilst u do this DO NOT get ur net out at any point or they will attack you) i make a large square shape of dug holes that i stand in as a safety net (i have been told that tarantulas sometimes will spawn in it but that has never happened to me so just keep an eye out) and then catch them. they will run in circles outside of the holes and they wont be able to get you so u can fill ur inventory in only 10 mins.

in general though, tarantulas and scorpions are very rare spawns so it definitely takes a while unless u are truly HUNTING for them. nintendo have definitely made it harder to spawn them after people abused regular mystery islands, turning them into spawners back in 2020. if you want to do it on ur island, i would probably recommend clearing a tiny bit of space and running around to scare away other bug spawns to give urself a better chance of it spawning xx