How can I selectively delete hundreds of SMS conversations?
Act fast to get quick results

Use SMS backup and restore and backup all your SMSes to local storage on device immediately.

Go to the app menu and choose restore. It will select the backup you made. Go to advanced options and select only restore backup since and you choose date before you got flooded by all these SMSes.

Assuming you don't have any important messages after the chosen date , you are good. If you have, follow steps in answer here to backup, selected conversations and choose those to be restored. It doesn't restore duplicates and at most you may end up having to manually delete a few messages

Alternatively, you can export the SMS backup file which is in XML format to your laptop, search for the common term in these messages, delete all those, maybe use a script to do that. Transfer the file to your device and restore to device.