So I'm spending about 60 hours in Istanbul (first time to Turkey) at the end of the month but since I get in later in the evening it's probably closer to 48.
In any case, I am curious about a few things.

From what I gather, Turkey is a mainly cash based society. While I don't intend to go nuts shopping (maybe a few souvenirs but no knock-offs) I've also seen/read that credit cards are accepted widely even in the Grand Bazaar and Spice Market. Is this true? Otherwise, I planned on bringing about $120 CAD (about $2400 Turkish Lira) for the few days I'm there.
The only souvenirs, I'm looking at bringing are the copper coffee pots, coffee, tea and a few small things like candy and spices. I'm travelling with carry-on only so space is limited.

Another thing I saw mentioned on youtube was that if you bring in more than one mobile phone you get taxed because the authorities think you'll sell it there. I have an OG iPhone SE and cheaper Motorola. I can understand someone with flagship phones, but even these? My iPhone is my travel phone because of the smaller size and the Motorola is my main phone. My iPhone might even be able to pass as an iPod touch, but I'd rather be honest and not end up in jail or with a massive fine. I really just have it to check emails when near wifi and likely wouldn't even leave the hotel.
Anyways, is there any truth to this?

Lastly, I've seen market tours with guides. While I'm usually one that prefers to do things on my own, is this really worth it?

My plan on things to do (in no particular order) is: Hagia Sophia, Blue Mosque, Basilca Cistern, the Hippodrome, the 2 markets, the boat tour over to the Asian side and if there's time maybe the Istanbul Archeological Musuem.

Anything I miss?
I haven't seen anything mentioned on the Asian side, but am open to suggestions that aren't too far from the boat/ferry on foot.

I know it's a tight timeline, but I'm trying not to overload it with too much stuff or things that mean waiting in long lines.

Thanks!

Also forgot to ask, is there public transit to the airport in the early hours or are cabs really the only way?
If it's only cabs, am I more likely to get a set price if it's booked through the hotel (albeit pricier) as opposed to try to do an uber myself?
Thanks!

When I visited I also went to Hagia Sophia and the Blue Mosque and liked the Hagia Sophia much more. The Blue Mosque was okay but not as great as I expected. What I can recommend is the Topkapi Palace https://muze.gen.tr/muze-detay/topkapi .

Credit cards are widely accepted in Turkey but be aware that American Express, Diners Club, Discover, etc... might not be accepted everywhere. If you have a Visa or Mastercard, no issues on that.
However, note that small boutique stores, etc... might not accept cash or might ask a surcharge for credit card payments.

They usually don't check foreigners but they target Turkish people who are trying to bring multiple phones to avoid import tax. The enforcement is similar to Mexico or those Latin/South American countries which have similar laws where tourists/visitors are not thoroughly checked.

Don't waste your money on tourist trap tours at all. You could everything all alone without any issues.

Maybe add Galata Tower and the Dolmabah�e Palace to your list to do in the European side. The Archeological Museum has lots of history and if you are into it, you would like it.
In the Asian side, since you don't have a lot of time, my only suggestion would go visit the Maiden Tower which is accessible by boats from �sk�dar. Alternatively boat services from Galataport shopping mall on the European side are also running.

Unless you spend a lot of time at each sight, your plan looks reasonable.

Yes, there are municipality buses as well as coach buses ( Havaist ) that run from Istanbul Airport 24/7.
Sabiha G�k�en Airport also has 24/7 public transportation.

Also bear in mind, usually hotels, or those tourist trapping taxi/Uber drivers etc... would ask double the price of what a taximeter would cost.

1. dont want to speak with authority about the rules there but taxation on actual, personally-used phones seem asinine. i've also never ever once been scrutinized entering with phones for the 20 or whatever instances i've entered that country.
2. nothing special here with respect to means of spending. cash for convenience (e.g. small purchases) and cards for big purchases.
3. whether you need a guide for markets depends what is your objective in the market. if just walking by to see it, you probably dont need one. that said, i don't see their appeal at all: they're gigantic, crowded, and if you've seen a few markets like that around the world there's nothing awesome or outstanding about these ones.
4. the country has incredible food and a trip to the asian side is worth it to sample some restaurants. also, just the act of taking the ferry across the bosphorus is in itself a worthwhile thing to do.