You are not the 1st asking this question... Actually you can reach 20-25% of efficiency after optimization, but you will not solve the problem of toxicity. See for example what was achieved with our Viromer RED on the attached file.
A great alternative is to transfect mRNA (if possible modified mRNA) instead of plasmid DNA. Either obtained by IVT but better if you can buy it to a supplier.
If you want at least to try our Viromer RED, please let me know, I can organize a free test sample for you.