I get what you are trying to do but you may be thinking about this the wrong way. If the idea is that you want all the satellite channels you can receive to be viewable on all the TV's in you home, and that you can select the channel to view from a cable-like guide interface, then here is an approach that will work, particularly if you have room for multiple dishes.
First you could equip each dish with a dual output LNB and run two coaxial cables from the LNB outputs into the house, this helps you avoid scheduling conflicts.
Inside the house the cables should connect to DVB-S2 capable PCIe tuner cards, that are installed in a PC. Several companies make these, but TBS seems to be the most popular in North America. I would get quad input cards if you can afford them, to support more LNB's and again to avoid scheduling conflicts. The PC itself can be running Windows or Linux, and which you choose will in part determine which PVR software you can use.
Then on the PC you will need to run what is called backend software, on Linux you can run Tvheadend or maybe MythTV (Tvheadend seems to be easier to use), while on Windows you could run MediaPortal or NextPVR or something similar. This software will stream your live and recorded programs to your frontend clients.
Then at each of your TV's you run a media center PC and on that you will probably want to run Kodi or something similar. Alternately, if using MediaPortal I think they have their own frontend software. You connect the TV to the HDMI output on the media center PC and go into Kodi to watch TV. In Kodi you can either select a channel to view live, or you can view a recorded program from the backend, or you can view any video stored your hard drive or a network share. The program guide in Kodi can look very much like it would on a cable TV system, if you set it up that way and choose a Kodi skin that you like.
The way this works is, when you want to watch a program from the satellite, you go into Kodi and go to the Live TV menu and select a channel to view - you can even do this using a remote control if you like. Kodi then requests the channel from the backend system, and the backend system instructs the satellite tuner card to tune in that particular channel and then it streams it to the Kodi software or other frontend software, which in turn sends it out the computer's HDMI output to your TV. Of course this is an extremely simplified overview of the process, but at no point do you need to convert the signal to ATSC. In fact technically you don't need to play the received signal on a TV at all; it will play on whatever display device is connected to the computer.
The computer running the backend software doesn't have to be very powerful although the power supply needs to be large enough to power the tuner cards. But a several year old desktop computer often works just fine. The computers that are acting as frontends don't need to be super powerful either BUT since some of the signals on the satellites are in 4:2:2 format, which can't be decoded by most GPU chips/cards, you will need a frontend with a decent CPU since it will do the work of decoding those channels. If it were not for that issue you could probably get away with using something as small and cheap as a Raspberry Pi 4. but if you try doing that you will find that on some channels your either get no video or you'll drop a lot of frames. By the way you can run a backend and frontend on the same computer, but if you do then it has to be powerful enough to run the frontend software, so your old Windows Vista box probably won't cut it in that case.
I don't know if you'd be interested in going that route but media center PC's may not be much more expensive then ATSC modulators, the picture quality will definitely be better, and since the signal is being sent digitally you don't need to worry about coaxial cable loss. If the tuner card is getting a reliable signal then that's what you'll see at the TV's, provided you are running gigabit Ethernet in your home (I don't really recommend using WiFi in this application although I'm sure some people do). My suspicion is you'd get pretty much everything you are trying to achieve and more, just not in the way you're trying to do it now. Also, if you buy ATSC modulators, at some point ATSC3 will become the new standard and then new TV's may not have the capability of receiving the older ATSC standard anymore (do new TV's still have the ability to receive NTSC? If so I'd guess it's only because some people still connect VCR's and watch their old tapes!).
There are a couple of downsides (besides the initial cost if you have to buy everything new rather than repurposing older equipment), one is that it will take you a while to figure out how to set everything up, and the other is that if you have any users that can't figure out how to operate a cable box then they won't be able to use this either. As I said you can use a remote with Kodi but the setup isn't exactly straightforward. Another little fun thing is getting guide data into your backend software; you can do it (for free even) but it requires additional software. So there are all these little things you need to learn how to do to make it all work, so if you get frustrated easily and want everything to just work now then this is not the solution for you. But if you can persevere and figure it all out, I suspect you'd like the result much better than what you are contemplating now, unless for some reason users absolutely have to be able to change channels as if they were watching OTA TV channels off of an antenna.
