I've had 2 different assistants over the years. Unfortunately, my experience was that they were overpaid for what they did (data entry, minor investigations), and while I appreciated saving some time it never sat right at the end of the month when I calculated how much they did and how much I paid them. Both were very good and very flexible, but there is only so much an assistant can do for a solo residential appraiser. I've been without an assistant for over a year now, and I don't regret it. I've automated everything I can think of (like DataMaster), and have been very organized in terms of saving useful information for later use that will save future time (e.g. quotes from zoning docs for common non-conforming properties that I will use again and again). I've also spent years developing a massive spreadsheet that I track nearly all of my adjustments. While I don't copy adjustments from prior analyses, it serves as a decent check when I run into problem appraisals where the data may not be making as much sense as I would like (usually due to minor market fluctuations or odd comps), which saves time by pointing me back in a more probable direction. The vast majority of my time is spent developing and communicating appraisals, something an assistant cannot (or should not) do. I'm to the point where the amount of work I could pass off to someone else would save me only minutes, not hours. YMMV.
Spend some time and think of every redundant thing you do, and if you can improve on it. Also consider and challenge all of your processes, I bet you'll find something you can do that will save time. And tech investments pay off!
Like the OP, I feel as if I've pretty much hit a ceiling, outside of getting higher paid work. But, I like my ceiling.