The lack of fertilizer in combination with reused soil is probably the main factor behind their struggles. The off-color, yellow hue to the foliage is due to either nutrient deficiency (mostly nitrogen), over-watered roots, or both. (Water can flush-out some of the soil nitrogen each time the container is watered or rained-on.) Do the pots have drainage, where excess water can freely exit the pot? If not, heavy rains or over-watering will drown roots, and since they won't function properly, that impacts how well they can supply the foliage with needed nutrients.

As potting mix ages, it compresses more, reducing the pore space between particles that let roots "breathe" and get oxygen. Over time, smaller and smaller pore spaces lead to stressed roots that stay wet too long after watering, which can be harmful to drainage-sensitive plants like basil. While basil isn't very tolerant of getting too dry, it also won't tolerate having roots be too wet too often.

Some potting soils also come with a starter amount of fertilizer built-in, which wears out over time, meaning future plantings can require supplemental nutrients in the form of fertilizer. Whether you opt to use an organic fertilizer or not is up to you, as long as it provides the basic nutrients (nitrogen, phosphorus, potassium, or N-P-K) and ideally some micronutrients like manganese, magnesium, and iron. Time-release granules might be more convenient to use than liquid-applied fertilizer, though this late in the growing season, either would be better than nothing so the foliage can green-up and the plant can better regrow after some harvests.

If you do not want to replace all of the potting mix in these pots next year with fresh mix, at least supplement it with some new mix blended-in with the old, which will help it regain some lost pore space to the benefit of root health.

The minor amount of browning on the leaf tips is hard to see clearly, but it's probably due to environmental stress (like root moisture levels) combined with the periods of high heat we experienced. If a minor fungal or bacterial leaf spot infection, all you can do is snip off the individual leaves with symptoms, and avoid getting the leaves wet when watering to reduce the risk of the infection spreading, since spores tend to splash around to new leaves in rain or during watering.

Miri

I read online: . You can use a 5-10-5 (nitrogen-phosphorus-potassium) water soluble fertilizer.

Is that the ratio you recommend?

The exact ratio isn't super important, though proportionately more nitrogen (the first number in the N-P-K) would be beneficial, such as (just an arbitrary example) 10-5-5.

Miri

No, you do not need to remove the yellow leaves, as the plant may pull resources out of them to reuse in other growth before shedding them.

Miri

The more recent leaf damage/discoloration is hard to diagnose, since several causes can create similar symptoms, but it might be due to the change in weather (basil doesn't thrive in cooler overnight temps), mild fungal leaf spot infection, and/or a pest like spider mites or thrips. This late in the season, a pest or disease is not seriously damaging or worth treating, especially since the final harvest of basil plants will happen soon. Regardless of the cause of yellowing in this case, yes, the leaves should be safe to eat as long as they're well-rinsed (as home produce always should be before eating or cooking with it). As our nights get cooler, monitor the plant's watering needs carefully so it's only watered once the soil gets dry enough, since it will be easier to over-water plants in autumn because the soil won't be drying out as quickly as it did during summer.

Miri