Mark of Sakiko or Bear Umbra might do some work, mostly to enable you to play a lot more stuff? Belt of Giant Strength seems synergistic, it stacks with your +1 counters. Crystal Shard is a pretty good way to get your Commander back in hand, but it can also be used to punish any player who dares use all their mana up (some players are really bad at tracking stuff like this, depending on the playgroup). Rogue's Passage is an interesting way to guarantee the survival of your Goose. If you want more mass removal I have had some luck with Mass Manipulation, it's hard to cast but the effect is pretty strong if you steal more than 1 thing.

I'm not sure if your Goose gets big enough or not, but Tempting Licid is a decent Lure effect, and you can technically attach it to an opponent's creature that attacks (and remove it after blockers are declared) to either wipe out blockers (letting you/others get in) or get a creature killed, you can also use it to put a Lure on something you don't care about so The Goose can get in unblocked (you can save the Licid).

When it comes to ramp and fixing, I find 2 colour decks are extremely forgiving, and having one of those colours be Green makes it even easier to build. One of the annoyances of Enemy Pair decks (like Simic) is that their aren't as many dual lands to choose from, but their are still a few budget range dual lands that come with basic land types, these really pop with stuff like Nature's Lore, which can find a non-Basic Forest, enabling you to get your colours quickly and consistently, but IMHO it's really hard to not hit your colours in a Simic deck. It's really annoying that the only 'budget' typed dual I can think of is Breeding Pool, but that's a long way from budget, and you're only in 2 colours.

If you like +1 counters and drawing cards you might enjoy Generous Patron, if you're desperate you can give your own stuff the counters, but if you want cards you can do so (and still have a body). Another card that might be generally useful is Skullwinder, you can use this to generate political leverage, possibly getting to choose what the opponent will actually get back. Cultural Exchange is an obscure card that can very much take over a game if you time it right, ideally you use it after people have already wiped and hopefully don't have another at the ready. Mass Diminish might be better than Frogify, note that it lasts until your next turn and affects all of a player's creature, yet still comes with Flashback to boot. This is very good with something like Tempting Licid. Transmogrifying Wand might not look very good, but being able to remove the 3 most annoying creatures for a grand total of only 6 mana is pretty sweet, it's also a great deterrent piece once it's out and still has counters, people can't play key creatures until it's dealt with or out of counters. Return of the Wildspeaker is a good way to refill your hand, it's among the best of it's kind, but Momentous Fall is better if you're being wiped or something.

THX a lot for the ideas, these sound great! Yet, the biggest question: What to throw out?

I took a look and found the following cards that I think might not synergize especially well with your deck. Murkfiend Liege seems a bit 'meh' unless I'm missing some very powerful tap abilities, if you want Vigilance you could use an equipment, I find creatures tend to get wiped more often than artifacts, and stuff like Haunted Cloak do not attract a lot of attention. Consuming Tide seems like an odd card, Simic often has a big board, I feel like your deck should be trying to have a bigger and more threatening board than most other decks, and if the opponent redeploying would leave you behind, that will be a problem I'd think. I think I'd rather have Mark of Sakiko than Emergent Sequence, my issue is that while Sequence is better very early, the creature will die very, very easily, so even though it's land ramp it's not very good, if you're sold on a spell their is Nature's Lore, or creatures like Wood Elves/Farhaven Elf that won't take mana with them. I think their is better card draw than Tezzeret's Gambit, it's not that it's terrible, I'm not sure Proliferating one time is worth very much, I'd much prefer to have Momentous Fall to help rebuild when I'm wiped. I'm also not a huge fan of Thought Vessel, Simic decks can get away with running very few artifacts, making them less vulnerable to artifact hate. I am a bit clueless why you're running Cerulean Wisps, though I could see it being a pet card. I'm not sure what I'd replace it with, but I struggle to see how it could be impactful. I think I'd happily replace Unsummon with Crystal Shard, even if it is another Artifact the effect is pretty useful. Similarly, I think I'd replace Frogify with Mass Diminish, Frogify is a fun and powerful enough effect, but Mass Diminish has a much larger impact in most games, and if it gets turned around VS you somehow your deck uses +1 counters and won't suffer as much!

I hope this gives you some food for thought!