First, this is not my primary area of expertise, however I ws an EE in a former life, and I have recently worked (as a contractor) at a startup that made custom chips, so here is a couple things that might (hopefully) be useful.
1) MVP; you might consider implementing your idea using FPGA first. You won't need custom silicon to prove/develop the concept, and I imagine having this working prototype will make fundraising even easier. As to whether this is viable depends on your product, of course.
2) Things have certainly progressed since I was an EE, and there are now batch services where you can get chips made sharing a wafer with other folks designs. This may or may not be feasible depending on what process you require. You typically wind up with dice that you send to another facility that will do binding to the leadframe and encapsulation and testing.