I'll post some of my favorite flashes, so you can get an idea of what it was like
Here on 4chan we used to have "Touhou Tuesday," where every Tuesday people would post a bunch of Touhou-related flashes on /f/
/f/ is now dead for good, but it used to be a big part of internet culture a decade or two ago.

https://dagobah.net/flash/sound_test_in_touhou.swf
I really like this one as I saw it quite early on, and it really made Touhou look mysterious and interesting to me
https://danbooru.donmai.us/posts/412254
This one too

https://dagobah.net/flash/No_thank_you_featuring_Kaguya_Houraisan.swf
This one's catchy and based off a Miku song. Has some classic touhou memes and I really like how multiple artists all contributed to it and you can see their pixivs listed at the end. Really shows how the community was like back then.

https://dagobah.net/flash/bouncing_touhous.swf
a /jp/ classic

https://danbooru.donmai.us/posts/630099?q=kagiyama_hina+iosys
a classic IOSYS flash, you can click certain things during it for secrets

https://dagobah.net/flash/IOSYS_tohootomebayashi_loving.swf
https://dagobah.net/flash/IOSYS_tohotsukitoro.swf
https://dagobah.net/flash/IOSYS_toho5_OyomeNiShinasai.swf
https://dagobah.net/flash/IOSYS_toho9_SansuuKyoushitsu.swf
https://dagobah.net/flash/tou_take.swf
https://dagobah.net/flash/niji.swf
https://danbooru.donmai.us/posts/413986?q=iosys
https://dagobah.net/flash/IOSYS_Toho_Hatate_PV.swf
https://danbooru.donmai.us/posts/463106?q=border_of_extacy
https://danbooru.donmai.us/posts/494650?q=iosys
https://dagobah.net/flash/IOSYS_toho5_Usatei.swf
https://danbooru.donmai.us/posts/83635?q=iosys
https://danbooru.donmai.us/posts/377800?q=iosys
https://danbooru.donmai.us/posts/409995?q=iosys
More IOSYS classics

https://dagobah.net/flash/Touhou_Bad_Apple_PV.swf
You should've seen this by now

https://dagobah.net/flash/Tsurupettan.swf
DELICIOUS FLAT CHEST

https://danbooru.donmai.us/posts/8944324
The /jp/ dream

https://dagobah.net/flash/carameltouhou.swf
https://dagobah.net/flash/Someone_WAN_WAN.swf
https://dagobah.net/flash/Utsuho_Reiuji_and_her_egg.swf
https://dagobah.net/flash/Sound_test_with_Remilia_and_Flandre.swf
https://dagobah.net/flash/Hina_Face.swf
https://dagobah.net/flash/chennyanyan.swf
https://dagobah.net/flash/Grindcore_Drummer.swf
https://dagobah.net/flash/WE_ARE_JAPANESE_GOBLIN.swf
https://dagobah.net/flash/Touhou_Vodka.swf
https://danbooru.donmai.us/posts/369238
https://danbooru.donmai.us/posts/897919

a few other randoms

https://archive.org/details/kaguya-table
Kaguya Table Flip RPG

There's a whole bunch more I forgot or can't find, but I hope you enjoy them and the effort I put in to find them from my memory.
This was a big thing back in the day, now it's all tears in the rain....