You are right when you benchmark it to industry, like a production company. My suggestion is to benchmark it to other insurance companies in your country and abroad. Good luck! Information should be available in annual reports.

I think the following paper will help you out in this regard:

Effects of Financial Performance, Capital Structure and Firm Size on Firms' Value of Insurance Companies in Nigeria.

H Ayuba, AJ Bambale, MA Ibrahim… - Journal of Finance …, 2019 - search.ebscohost.com

These articles can serve as a starting point.

If your question is driven by the need to evaluate the value of equity capital absorbed by the insurance company, there are two ways to solve. The first method is to apply the method of successive iterations, which follows by successively discounting the expected income flows in the forecast period, starting from the last year and moving step by step to the valuation date. This method allows for the calculation of equity flows (FCFE) and the rate of return on equity (Re) or returns on invested capital (FCFF) and the weighted average cost of capital (WACC). The second method uses the Evans-Bishop formula to calculate the market value of equity capital: MVe = (FCFF - D×(Rd×(1-tax) - g))/(Re - g), where D is the total interest-bearing debt, g is the expected long-term growth rate of income streams, Rd - interest rate of borrowed capital.

More correctly,

"This method allows for the calculation of market value of equity using cash flows of equity (FCFE) and the rate of return on equity (Re) or cash flows on invested capital (FCFF) and the weighted average cost of capital (WACC)."

Your argument that debt/equity ratio is problematic for financial firms is in order. You can consider the use of cost of capital. If you take the weighted average cost of capital, it will serve well as a proxy for the capital structure of financial firms