First time need help please!

I’m a newbie grower and I have no idea if these are close to harvest or not lol. Not really 100% sure what I’m doing I’m a novice for sure.
Some of the leaves down the bottom are yellowing. I don’t use nutrients because when I did it looked like it was getting nutrient burn because it wasn’t the right ratio and in Australia I can’t find anything that has the right ratio of nutrients. I do PH balance the water and try not to over water. It’s currently on a 12 hour cycle 12 hours light 12 hours dark.
They are bubblegum and Og Kush auto flowers and they have taken much longer to get to this stage then they are apparently supposed to. I’ve had them going for probably 4-5 months. It is winter here is Australia as well so I’m. It sure if that makes a difference.

Any guidance would be appreciated.

More important is to maintain PH and moderate input with the supplements. You can use a wide array of ‘ratios’ and still get the needed supplements to the plant. Monitoring the TDS out is always a good idea (ppm’s).

The one plant looks like it has stalled out; likely due to Phosphorus and possible N excess. It would be wise at this point to have a decent meter that can tell you what salts are coming off of the soil layer.

I’m so very new to all this I’m not sure what you mean by ppms and TDS. The reason I stopped with the nutrients is because the leaves were going brown and I was told that was nutrient burn off this forum and I needed to PH flush the plants and I was scared it would do it again and I don’t find anything with the right ratios.
What kind of meter do I need?