Thanks a lot for playing the game and for taking the time to write such detailed feedback. I really appreciate it!
The achievement idea is actually something I've been thinking about. I'd like to add achievements in the future, and maybe even a global scoreboard or something similar to give players more long-term goals.
As for the checkpoint system, you're definitely not the only person who's brought this up. Because of that, I'm planning to change it. I don't think I'll go with unlimited saves, since I still want the game to reward players who complete runs with fewer checkpoints. The game already has a scoring and ranking system, so my current idea is to allow multiple checkpoints but have each one affect your final score or rank. That way, players who want the extra safety can use it, while those looking for the highest ranks can challenge themselves without relying on checkpoints.
Regarding enemies gathering outside the shop, that's actually something you can plan around. Try getting a bit creative with the tools the game gives you. For example, you can place three C4 charges a couple of tiles apart before entering the shop. If enemies gather while you're inside, you can detonate them as soon as you come out and clear the area. Or, if you're feeling brave, you can run into the middle of the group and drop a bomb. You'll lose about 50 health, but it can wipe out a large number of enemies. There are quite a few different strategies you can use depending on the situation.
Anyway, right now my main focus is adding a self-update system so players can update the game easily without having to keep coming back to the itch.io page every time a new version is released. After that's done, I'll continue improving the game based on everyone's feedback.
Thanks again for your suggestions, and I hope you keep enjoying the game!