To put a disclaimer prior to my review, I've played through this game on my desktop with a local .SWF file. So I'm not taking Ruffle or any other flash emulation software issues into account here.
It seems like you've taken notes on what didn't work and what to adjust with the first Endless Zombie Rampage game, and this is indeed a good step up from its predecessor. But despite saying that, this game is still fairly flawed and just isn't designed very well. I'm not too sure on what your time frame was and for how long you were able to work upon the sequel, but it feels like Endless Zombie Rampage 2 was made to be artificially continuous while providing as little as possible to work with. This is common in flash gaming, sure, but to me this just feels intentionally subdued or rushed.
I'm personally not a fan of games that go on forever, so normally I wouldn't spend so much time playing a game where "endless" is directly within its title. But since technically there is some sort of campaign included in the game which you can complete, I've only briefly poked around in the survival mode in favor for spending most of my time within the main endless rampage mode which ends upon reaching the fortieth day. You can progress onward to that point at your own leisure, but you'll still have to eventually conquer four boss battles in the main mode prior to reaching that ultimatum. Bundle that with the enemy variety that increase in difficulty, as well as having to juggle around objectives in the meantime, the result still offers a busy environment regardless of the relaxed pace. But in terms of complexity it's all still very surface level.
So, what did I like compared to the first game?
• The variations of enemies is condensed. I would rather fight a smaller amount of distinctive monsters as seen here, versus an array of the same ones with recolored sprite work and different hit point stats. Every enemy now feels unique enough with its approach as well. This however could be elaborated upon further, see my suggestions list for more info.
• There's a smaller arsenal to work with. Endless Zombie Rampage just had too many weapons; many of which were useless and/or a waste of points. But with this new minimalistic approach, investing in new equipment is less punishing and it allows players to be able to experiment with what works for them without having to worry about a bad buy becoming a softlocking mistake.
• Your artwork and asset design here is top notch. It may not be the most detailed as this is a top down shooter, but your graphical work here has a fine balance between personality, detail and readability. You were definitely on top of your art style during this era of your browser game development career.
And what did I not dig in this game?
• There's just not enough general variety to maintain interest. I do admire the effort, but this game just feels mundane compared to other zombie-based flash games like The Last Stand 2 or Rebuild 2. Maybe it's a bit unfair to compare this game to those ones as they're all different, but my point here is that this could have been an opportunity to expand upon the first game and have Endless Zombie Rampage 2 include more gameplay elements beyond the repetition of point and shoot. There's really nothing to stimulate your mind here, and I feel like I've seen everything that I needed to twenty-ish days in the endless zombie mode.
• Weapon balancing is still an issue; because I've ultimately found only three weapons to be useful... and these are the assault rifle, auto shotgun and flamethrower. The rifle does lose its potency when the bigger monsters get introduced, but going all out with a fully upgraded auto shotty and flamethrower will guarantee to keep you alive until the end of the game. Plus with the amount of enemies that will eventually begin to clutter the screen, why would you pick a weapon that requires you to be mashing the fire key as you attempt to multitask?
• The difficulty... this game is just too easy. I managed to reach my conclusion without using a single first aid kit. It's just too lenient with no challenge, and as a result Endless Zombie Rampage 2 gets boring pretty quickly without that sense of satisfaction of overcoming any obstacles.
• The soundtrack isn't great. It just sounds like a subpar remix of the music from your Eternal Red game spliced amidst some generic beats. It just doesn't flow very well, you know?
What I think this game could benefit from?
• It may be far fetched, but having more environmental things to interact with or attend to would have been nice. The level design here is non-existent as it's just a flat parking lot with a small safe zone in the middle... it's just bland. I know it'd be ripping off Dawn of the Dead and Dead Rising, but how about having like a small mall or department store that you'd have to work on securing with different busy work and different objectives instead? You can still implement the "at your own leisure" approach this way too, and the survivors you rescue could be given additional functionality in this sort of setting as well. There's barely any story being told here either, which while is fine, I think having a more interesting setting would give you opportunities to do social commentary / gags / some sort of humanization with your cast of characters.
• More different enemy variations; I've said that it's a good idea to have lesser copy and pasted enemies and have more unique ones instead. But if you want the main mode to last for as long as it does, I feel like there's room here to come up with some more monsters, or have the ones you fight against change in behavior as time goes on.
• Additional animation. Most of the zombies or mutants in this game don't have a visible attack, the humans really should be more animated beyond just a mere walk cycle. For the player character, some reloads would have been nice.
• Revise the weapons and give them some distinguishable traits. Like maybe the submachine gun could be faster to aim with than say a rifle, that's stronger but kinda slows you down? Also this would allow you to add more weapons that are unique. Also I get what you are attempting to do with the bullet spread, but that has to be nerfed. It's annoying to see the projectiles just fly all over most of the time in such irregular angles, especially while using the light machine gun with its enhanced upgrade purchased (that causes its discharge audio to peak uncomfortably, I might add).
... In conclusion, you can see potential with Endless Zombie Rampage 2, and for how it turned out it isn't a bad game whatsoever. But it just feels like shovelware that would be released for the Game Boy. I gave it a good thorough play session that lasted quite some time, but I think that apart from serving as either a distraction during work downtime, or just needing to pass time while you wait on someone or something, there's not much going for this game that warrants such extensive attention like I just gave it.
I'd still recommend Endless Zombie Rampage 2 if you haven't experienced it yet, but I do know there's better games under the Diseased Production brand that are more fun to play instead.