This corresponds to both Gateway (La Grande Porte in French), by Frederik Pohl (1977, winner of the Hugo and Nebula awards, as well as its first sequel, Beyond the Blue Event Horizon (1980). These are the first two books of Pohl's longer Heechee Saga.
You say that you only read the first book, so it may be that the French edition you read had some additional teaser material added at the end, drawn from the second volume.
Indeed, almost all of what you describe comes just from Gateway. There are Heechee tunnels under Venus*, and most of the action concerns the titular space station, where hundreds of partially preprogrammed Heechee ships are docked. From the Wikipedia plot summary for the first book:
Gateway is a space station built into a hollow asteroid constructed by the Heechee, a long-vanished alien race. Humans have had limited success understanding Heechee technology found there and elsewhere in the solar system. The Gateway Corporation administers the asteroid on behalf of the governments of the United States, the Soviet Union, New People's Asia, the Venusian Confederation, and the United States of Brazil.
There are nearly a thousand small, abandoned starships at Gateway. By extremely dangerous trial and error, humans learn how to operate the ships. The controls for selecting a destination have been identified, but nobody knows where a particular setting will take the ship or how long the trip will last; starvation is a danger. Attempts at reverse engineering to find out how they work have ended only in disaster, as has changing the settings in mid-flight. Most settings lead to useless or lethal places. A few, however, result in the discovery of Heechee artifacts and habitable planets, making the passengers (and the Gateway Corporation) wealthy. The vessels come in three standard sizes, which can hold a maximum of one, three, or five people, filled with equipment and hopefully enough food for the trip. Some "threes" and many "fives" are armored. Each ship includes a lander to visit a planet or other object if one is found.
The narrative alternates in time between Broadhead's experience on Gateway and his sessions with Sigfrid, converging on the traumatic moment near the black hole. Sigfrid helps him realize that, due to the gravitational time dilation due to the black hole's immense gravity field, time is passing much more slowly for his former crewmates and none of them has actually died yet. Broadhead, however, concludes that this means that they will still be dying when he dies in several decades, with Klara still believing that he betrayed them to save himself.
Also embedded in the narrative are various mission reports (usually with fatalities), technical bulletins, and other documents Broadhead might have read on Gateway, adding to the verisimilitude. The economic side of living at Gateway is presented in detail, commencing with the contract all explorers must enter into with the Gateway corporation, and including how some awards are determined.
The last two of your points concern what happened to the Heechee after they disappeared from the galaxy, leaving their tunnels on Venus and stations like Gateway full of operational ships.
The Wikipedia article for Beyond the Blue Event Horizon is much sparser. However, the novel does have the last two points you mention. It is theorized by the human characters (with assistance from powerful artificial intelligences that the protagonist's wife programs) that the Heechee have hidden inside the event horizon of the black hole at the center of the galaxy. And the very end switches to the point of view of a Heechee captain, watching the exterior universe from their black hole hiding place. The time dilation effect means that as the captain walks across the room and looks out, human civilization speeds by, and this is narrated vividly.
*Not Mars; thanks to JRE for the correction.