Based on your query, I would recommend using Twee. This tool is designed to assist English teachers in creating content for their lessons, which includes exercises related to the teaching topic.
One of the key features of Twee is the Vocabulary Builder. This feature allows you to generate vocabulary lists related to your topic and design fill-in-the-gap and open-the-brackets exercises. This could be particularly useful for teaching letter sounds, as you can create exercises that focus on the sounds of different letters.
Additionally, Twee offers Activity Ideas which can help you generate listening, speaking, reading, writing, and grammar exercises easily. This could be beneficial in reinforcing the learning of letter sounds through various activities.
The best part is that Twee currently offers free early access beta, making it a cost-effective solution for your needs.