You don't really give us enough context to help you. What is your pricing, what does the software do, who are your target customers, why are they quitting, how do they find you? Do you have a trial, what's your typical trial to conversion %.

But first question is what was your plan when purchasing the company? SaaS companies typically have overvalued discounted future cash flows compared to other sources because they are much easier to make better. If you reduce churn by 50% or increase prices by 50% you can have substantial impacts on the business.

Second question is what did you buy?

When purchasing an app you are typically buying customers, brand, employees, and software. It sounds you rewrote the software, so that means you bought customers, brand, and employees. Why did you pay what you did for those items? This will give you some ideas about what to leverage to improve your situation.

What are you and your partners skill sets? Development, marketing, operations, customer support etc...

Have you asked people who churned why they are churning?

Typical plans are things like
- reduce churn (app is losing customers due to x, if we fix x this company is worth far more)

- increase price (increases prices, this app is under price compared to competitors due to pricing error, or it is underpriced b/c it lacks feature x or y that competitors have, if we add that we can increase prices)

- increase # of users (app is poorly positioned, or is marketing badly or not marketing, we think we can acquire customers for x and make money, there is a market previous owners didn't go after)

- reduce expenses (app is currently using onshore customer support, if we move that offshore we can reduce expenses by 40%, or we are paying someone $100/hr to support this ancient software, if we rewrite it we can find contractors for $50/hr)
(and of course others like bundling, adding features to upsell)

Your options are the following.

1. Do nothing, sometimes you screw up in really expensive ways and the way to make the most of this investment (especially if you have limited experience in ways to help the business) is to just collect checks, pick the low hanging fruit, and write it off as very expensive lesson. Given current churns, margins, expenses and say a 25% discount rate how much are the current cash flows worth.

2. Sell it. Selling it might be worth more than the future cash flows because the buyer has skills or sees something you don't.

3. Build the business. You have the options of reducing churn, raise prices, bring in new users, or reduce expenses. Given your description it sounds like the expenses are minimal so I'd focus on what of the first 3. This is going to be a lot of work, and if you're working 70hrs a week you probably don't have time for it, so you might have to make some hard choices about what you value. Money, security, career, family, health. Something is going to have to be sacrificed to get this done, if it was easy the previous owner would have done it. Make this choice intentially.

To reduce churn, talk to your churners, ask them why they are churning, fix it.

Look at prices, view competitors see what they're charging. If they're charging more you should charge more. Talk to your users, see if there is a service or feature they're missing and wish they had.

Ask people who are thinking about purchasing your product why they are or are not purchasing.
Look into how people have grown shopify apps before, try an email campaign, if the LTV is right try cold calling, are there influencers you could reach out to, try SEO.

No matter what you do it's going to be a rough time, building something is hard, and I wish you the best of luck.