Basically these days there is lots of concentration done over designing phase. Your company following waterfall approach, which is only to be used when you don't expect any change over a period of time for the S/W or application you are developing. But so far as you have understood for the acceptance of change, it would surely help you in the long run. As you said your customers do give immediate feedback, so you should undoubtedly go for agile approach. So far as learning for such models are concerned I would like you to suggest to read once Software Engineering by Pearson.

If you still have any confusions, get in touch with me. I would like to help you out and support your firm at my best.

According to me, entrepreneurs do need support but it could be either in terms of knowledge or in terms of money only. As a startup entrepreneur do not have expertise in terms of all the aspects so they do need support of the person in the respective field. As and when time passes they surely become independent and can sustain in the industry for a long run.