The goal, practically speaking, is to get the radon in your home low enough (I'll prefer the lower WHO level to the higher US EPA level on that front) as efficiently as possible (because radon fans run 24/7/365 and thus the less efficient they are at doing their job, the more you pay.)
If you have a radon system and don't have an active radon monitor so you can know the radon level in your house (which is the important thing) rather than just the suction in your pipes, you're operating under the constraints of an era when you could not get such a thing for less than $200 (2023, yes it will go out of date, but it's not $11,000 as in earlier times) and were at the mercy of paying for a radon test and waiting weeks or months for lab results to know if your system was working as designed, or not.
In those times, the proxy of knowing the suction value in the pipes when your system was installed and tested and hoping that if the suction remains the same the results will too was the best you could do, with occasional tedious lab tests to verify.
These days, you can know, directly, what the radon level in your house is on a daily or even hourly basis, for years, for the price of a few lab tests.
You can put a far more powerful fan than you need on your system and waste power. You can also (evidently you have) put a fan that is more optimized for high suction than high flow, or one more optimized for high flow than high suction. You can get fans of roughly equivalent air movement potential that are vastly different in efficiency (power consumption to perform the same.)
What you need, in terms of suction and flow, depends on the conditions under your slab - if you have a rock layer and/or enough pipes that let air move easily, low suction and high flow is good, even if there is clay soil, but not in the rocks under your slab. If you have clay right up to the bottom of your slab, you need higher suction. If you only have manometers (or a single manometer) on your suction pipe, you don't really know how well that suction is being distributed under your slab. If you have monitoring manometers distributed around your slab, you'd know - but if you don't, in 2023, a radon monitor will tell you directly how well the sstem is performing, rather than you having to take suction as a proxy for the system working.
If you are of the "hire a contractor" mindset, a well-equipped contractor can temporarily install an (expensive) variable flow and suction fan to determine what your house needs while monitoring spots around the basement floor with expensive electronic manometers, and likely their own active radon monitor these days, and then recommend a less expensive fan that meets that suction and flow, ideally at the lowest power use. If you are doing it yourself, you either spend a lot of money, or you bet on a lower cost fan you think will work, while being prepared to change that if it does not work well enough after all. Since you already bought a replacement fan, what you need is radon results, whether by slow, expensive once you do a few, lab tests; or by an appliance that is pretty affordable if you have a radon problem to resolve and keep resolved.
Fan intake diameter is generally mostly about flow. A vacuum cleaner will out-suction a radon fan, but it can't move nearly enough air to get the radon out from under your house, and it's horrendously inefficient and loud. A box fan will move more air, but can't suck very well at all. A radon fan seeks to suck well enough, with enough volume, to do the job. Some also try to be efficient, others don't bother. You'll pay the power company quite a bit more over the fan operational lifetime for the others.
A larger intake fan will generally move more air volume, but if you are necking it down to 3" pipe the numbers for 6" pipe may be wrong. Since you have two pits, I wonder if you could increase at least to 4" where they join before the fan. Using two fans (one for each pit, on separate pipes the whole way) would be another approach, since you've clarified that you are more interested in lower radon numbers than the cost, right now.
Note that I'm not recommending, nor will I recommend, a specific radon monitor - there are several available in 2023 at US$200 or lower, you can shop among them and make your own choice, or decide that you want one of the ones that costs considerably more, or keep paying for lab tests - suit yourself on that front. It is good to know they exist at not-unthinkable prices.
