How to remove flowers efficiently in ACNH?

I had a similar problem. Earlier in the game when it rained almost every day (and I had an islander obsessed with watering everything) flowers took a large portion of my island. Here's how I cleaned it up in a decent amount of time:

1. A lot of shovels
2. Carry a trash can
3. Place the trash can near your group of flowers
4. Dig up all the flowers you can
5. Dump them into the trash can
6. rinse and repeat steps 3 - 5 until you clean up

It still took about half a day to clean everything up.... but it was doable, put on a podcast/movie marathon/netflix/etc. and just go at it.

Something else you can do is employ "best friends", as they are also allowed to dig up things. So gather some friends, and make an event out of it (prizes for most dug, payment of bells/nook mile tickets/other stuff).

One more tip to prevent the flower outbreak. Make a design pattern that is just a full grid of transparency, you can place this to surround the flowers. They won't spread, and it still looks like normal ground. You can also use construction to add various paths... as long as it isn't plain ground flowers won't spread.