Wondering if it’s possible to play single player mode but keep building my character and collecting gear

I recently started playing Diablo 1 again after playing it originally in 97’. So it’s like I’m new to it all over again but wanted to know if in Single Player it’s possible to max out character levels and gear but in Single player mode enemies don’t respawn and I’m assuming after beating Diablo himself the game is basically over? A new game will be made and all the gear collected will be gone on the new game character. Is this correct? Would what I want to do be better done In Multiplayer mode? Hope im explaining myself properly