I am transiting through AMS on an unprotected connection as I booked an award flight. Arriving on SQ324 at 7:15am and departing on BA8496. Without delays it's only a 2 hour 45 min connection and SQ says they can't interline for checked baggage. So I will have to go through passport control, get my baggage at the carousel, drop my bag at the BA counter, exit through passport control again and then if all aligns boards my flight. I will have wifi on the SQ flight so worst case I could change my BA ticket to a later flight in the day. How doable is this?

Allowing an hour to get your bags to check in before BAs cut off of 45 or 60 mins before departure suggests you have about 45 minute to an hour to play with.

AMS can often be quite slow with both immigration and baggage.

It's also an airport where the BA bacg drop / check-in deadline is 60 minutes before departure so if your SQ flight is ontime you'd have 1 hr 45 mins to complete all those formalities.

If the BA flight is an award flight then same day change isn't available to you.

I am regularly waiting for passengers on SQ324 as an airport rep. This flight is late almost every day. Not much, but if it is only 10-15 min late and lands at the infamous Polderbaan (taxi time 15-20 mins to the gate), it is quickly 7:45 - 8:00 AM before you are on your way to passports/luggage. At this time, it is very busy at immigration as many long distance inbound flights arrive at this time. So getting through passportcontrol can take a while, also luggage delivery may take some time. When you exit from customs, you need to go up one level immediately to departure level, BA checks in at row 19 most of the time which is right above your arrival level. You are booked on a BA flight to London City, but at the same time they are handling Heathrow flights, it can be busy at BA check-in. Lately, i have seen only 2 agents at BA check in as there is (like everywhere) a shortage of staff. Please check in online for this flight. You will still stand in line though to drop off your bag. And finally, you indeed have to go again through security and passport control. Plus quite a walk to the gate as BA to City airport usually leaves from the end of the D-pier.
Now to answer your question: is this doable? it will be tight but it is doable. You may need the help of some fellow travelers who let you go ahead of them in the line though. Personally i would look for a later flight to London (Heathrow or City) to have much more ease of mind

That is true, I should clarify that the SQ flight is an award booking but I paid cash for the BA segment.

Update: I checked in for SQ at a counter and they managed to check my bag to LCY interlining with BA! Nightmare averted