So much this. There is such a thing as YAGNI in the early and tight days of a startup but tech debt is something that needs to be constantly addressed, and you as a manager need to trust your team to put in the proper engineering as a solution grows. Is quick and dirty fine for a prototype or MVP? In theory yes, but understand that a lot of devs have seen that turn into the final product (with customers on it within days of the "prototype" being finished in some cases). I've seen it with some of the worst codebases I've ever encountered, and those "prototypes" and "MVPs" were never overhauled with a scalable architecture, and the startups end up running out of steam because they did not invest in their product codebase, or better, have a sustainable pace of development that allows the solution to constantly remain high quality.

Don't have the runway for that? Are you managing the startup as well as you could? Why did you tell your investors that you could do it in five months of dev time?