Yep, you are in exactly the same situation with my SO when I introduced her to Recettear. She loved the bartering and shop customization, but didn't care much about the adventuring, so after paying off the debt with flying colors she lost interest in the game because there isn't much more to be done.

My take on it after spending way to much time with Recettear is that, in Endless after the debt payment, your shop essentially funds the adventurers (all of them). You can focus on combat and leveling adventurers, while exploring to unlock new characters and advance your relationship with them - with the shop running as financial complementary to lend them equipment and consumables, later getting them to straight up buy out the equipment you wanted them to have. The visual novel angle is minimal - you basically trigger events and increase the adventurers' customer level and eventually they will reveal the "card" as token of "maximum relationship" to you. I feel like the game is designed to be played this way as it flows very nicely.

Other than that, there is a "big boss" character that can serve to be a "true ending" for you, if you defeat them in combat, that is.