That's only 1 variable. It's been tried to disappointing results. You can double everyone's salary and it will barely make a dent. If you've had kids and a working wife, you know how bad it gets. The good jobs? They're in expensive real estate areas....nearly everywhere in the world. So houses are hard to find, daycare is hard to find, and support is terrible once you have a kid.
Once people get a job in the city, they don't want kids. You can't make someone want them. All you can do is make existing parents have more. So you're never going to get every couple to churn out 3 kids, but you can make families of 1 or 2 have 3 or 5. However, support is the main factor. Work is a massive burden and even if your wife isn't working, many kids is a massive burden.
Data has shown that incentives don't make a lick of difference, but support programs have. Sliding scale daycare is 1 great way to get people to have their 1st or 2nd kid. However, if you want anyone to have more than 2, you need support for older children: sliding scale daycare for young ones and free afterschool programs for older kids.
Additionally, you need to alter the culture for parenting flexibility. That's tricky because the gov can only set rules for gov jobs. My job is great about it. I can step out during the morning or afternoon for a school event and make the hours in the evening. Jobs that are rigid and stuck in the past are a huge burden for families. Even if you have a stay-at-home mom, the dad needs the flexibility to be involved with his kids' lives if you want families to go past 2 kids.
More money is not the main problem. It's a huge one, but the bigger issue is lack of resources. Daycares in my area require a 6 month application process. Spots go often. It's harder to get into daycare in some areas than many ivy league universities.
It's not cheap, but sliding scale childcare and LOTS of it, will help the birth rate, help families, and be a MASSIVE stimulus since nearly every dollar is paying for human labor for a segment of unskilled (no formal training) women, most of which are very young or very old. But even the programs for older go out to a diverse pool of individuals and are great investments in future citizens to build skills and a sense of community.
However, many gov have tried throwing money at the problem and hoping for the best. It's easy, but it just doesn't work well. It requires a lot of follow through since the resistance to reproducing is very complex and the causes are many and you can have infinite money. So long as you have a job or other responsibilities, it's a huge load. Reducing the load reduces the friction to the 3rd kid and beyond.
Put simply, you have to make it EASY to have kids. Nothing else really works.