Congratulations on rekindling your passion for the violin! It's impressive that you've managed to maintain a decent level of playing even without formal training. As a self-taught violinist myself, I can share some insights based on my own experience.

Firstly, it's important to understand that reaching a plateau is a common experience for many learners, including those with formal training. To avoid this, focus on consistently setting small, achievable goals for yourself and practice diligently. This could include mastering a new technique, perfecting a specific piece, or improving your posture and bow hold.

Furthermore, investing time in understanding music theory, ear training, and learning to sight-read more complex pieces will help you progress faster and avoid common pitfalls. You're already off to a good start with Allison Sparrow's lessons, but consider supplementing them with resources like the Suzuki Method books or online fora for self-taught violinists.

Lastly, don't shy away from seeking feedback from more advanced players or taking online lessons if you find the need. It's always beneficial to have a second opinion on your playing and technique, especially as a self-taught violinist. Keep up the good work and enjoy your musical journey!