I build API systems daily for myself + clients.

Also other on-demand non-API systems.

To provide you an answer requires a starting point + gathering a good bit of context.

Best if you book calls with me + other people who build this type of software daily.

Tip: Here's how I approach this for myself + clients.

Whatever code I develop, evolves through stages, which self fund the project.

In other words, every month of development must produce income the next month greater than last month's dev cost.

So if month one's estimate is $10K of dev work, this work only starts when there's a clear path to the $10K of work generating $20K of income next month.

My suggestion.

If you can't come up with a clear plan to have income grow fast enough to immediately cover all development cost, get a new idea.

Hint: Most projects I do for myself + clients at this point use a pre sale model.

So, someone (client, myself, many people) market the product at a discount for a small number of users to be in the alpha + beta stages of release.

Sometimes this can generate... well... substantial income...

This also gives a massive window of debugging, where development + debugging costs are covered.

Think of this as a Kickstarter project where money comes in, sometimes for many months or a year at a discount.

So if your service will run $100/month, pre sell a $1200 package for 2-3 years, rather than the normal year of coverage.

To do this you must be clear with participants...

1) There will be problems to wring out of code.

2) You must be clear you can code some sort of product in a few weeks, to provide some sort of deliverable.

3) Don't spend all the money, so you can do instant refunds for participants who loose patience with alpha/beta code quality.

This approach works super well.

There are indeed a number of off the shelf frameworks for the web platform and mobile apps. You should find a few by spending some time. First, identify a mainstream app that's close enough for your concepts like TaskRabbit or dolly or uber. When you are looking for it look for xxx clone script. Make sure the platform is available with source code. Many cases they can fit the platform to your requirement without starting from scratch. That would save around 80% of the development. It might be worth looking for a platform other than WordPress to create scalable platforms.

Look for wordpress plugins, rather than templates.

True, building an on-demand web services platform from scratch can cost you thousands of dollars. You can choose ready to launch or turnkey solutions like YoGigs for building the on-demand services marketplaces at very less cost.