I REALLY love this! The seamless way you can really feel your progression makes the game feel extremely addictive, and the relatively low consequence for death adds a chill-factor that is really uncommon for the genre. I really, really hope you flesh this out because I can see myself coming back to it again and again for quick, casual, mindless soulstealing.
If you do, I have two suggestions. First, controller support. Survivor games always feel so much better with one, and it's a lot more comfy than WASD. I ended up mapping the controls through Steam and it felt great.
Second is a spoiler for the last wave, so if any others care, skip to the next paragraph. ||Since the final boss can end your run and force you to start from zero, some kind of meta progression would make losing to the boss at least a little less frustrating. If you intend to keep the scope about this length, the meta progression can be entirely based around fighting cursed weapons directly, leaving the rest to the normal leveling progression. If the scope broadens, there are plenty of ways to expand that out. But as it stands, you're currently incentivized to guess how long you have before the boss comes and die at the last possible moment beforehand, because if you happen to get lucky and cruise through the basic enemies with some good upgrades early on, you might end up still VASTLY underprepared and you get punished hard. Alternatively, an approximate time warning before the boss arrives would help a lot, allowing for those that want to level more to die before getting run ended.||
Spoiler end! All in all, very impressive, and I look forward to seeing how you flesh this out and whatever comes next from you!