As has been noted, if you don't know why you're splitting the ground, you probably shouldn't. Where you place the ground plane connection depends a lot on where the currents are flowing in your circuit and which points 'matter'. Splitting the ground is kind of like creating a star ground point where they join, such that every ground pin sees as close to the voltage at that point as possible, with minimal influence from other currents. The primary reason you do this with digital and analog grounds is so that relatively high digital currents with fast edges aren't influencing the ground voltage seen by the ADC or sensors, and in this aspect, it doesn't matter much where you join the planes as long as there's only one place where they join. There are two additional considerations (well, probably more) - first is that you don't have a dedicated analog ground pin in this case, so you can expect some digital currents to flow into its ground plane. Since this is unavoidable, you probably want to aim to minimize the distance between this pin in particular and the 'star ground' point where the planes join, to keep the ADC ground and 'reference' ground of the star point as close as possible and minimize the voltage change on the analog plane created by these currents. The second is that this point becomes the 'reference ground' for your system, so in particular you want your ADC's ground reference to be as close to this voltage as possible, so placing it near the ADC makes sense. If you were to place it elsewhere, the power supply currents flowing will affect both the ADC and sensor ground reference voltages instead of just the sensors, and likely in a less symmetric way, too.
Analog have some pretty good posts on the topic that I suggest you read:
https://www.analog.com/en/analog-dialogue/articles/staying-well-grounded.html https://www.analog.com/en/analog-dialogue/articles/grounding-again.html and probably some others.
None of this is likely to matter using the ATmega's internal 10-bit ADCs, though, and simply splitting the plane to avoid digital currents causing ground bounce and the like is going to have the biggest impact, but even then with this level of precision it probably won't matter unless you've got some fairly serious currents flowing. Considerations of power supply and AREF are probably more relevant.
I would place the ground plane join directly at the GND pin closest to the microcontroller inputs that you are using for the ADC. I don't see the micro in either of these screenshots so not sure where that is, but it doesn't look like where you've put it. Widen/extend the plane so it covers all the analog traces all the way to their microcontroller input pins.