Is there a way to prevent the Messages app from sending long texts? It's generally a nice feature, but my current plan has free SMS which I cannot use when overstepping the character limit - instead, the "long text" will be sent as an MMS and charged as such.
The default Messaging app does not show how much characters you have typed/left for it to be sent as an SMS. So you can use a third party messaging app like Textra SMS.
When you start typing the message, after typing a minimum of 22 characters, you will see a counter showing you the number of character left for it to be sent as a single message(eg. 130/1 - Which means 130 character are left).
The total length of the a single SMS message is 160 in Textra app(which is mostly the normal limit of a single text message). Based on your limitation you can send multiple separate text message to avoid sending a single MMS based on the counter. I don't send lengthy text messages but AFAIK even if your type more than 160 characters it will still be sent as an SMS rather than an MMS. But it would cost you twice the amount for that 160+ charactered SMS message.