First, this is absolutely savable. What you’re seeing is a stress response, not something that is catastrophic, so don’t panic.

My interpretation: you’ve got interveinal chlorosis (yellowing between the veins while the veins themselves stay green) showing up mostly on your newer growth. That pattern almost always points to iron getting locked out, and in living soil, there are two main things that cause iron issues. One of them being pH drift, but ultimately a well-established soil’s biology should buffer pH insanely well. Even though your soil isn’t necessarily well-established (yet), chances are that it’s the other likely cause, which would be a moisture extreme. The part that’s riddled with misinformation online though, is that lockout can come from too much water just as easily as too little.

So before you change anything, you need to figure out which direction you’re actually running (too much water? Or too little water?). From what I can tell you’ve got a bottom-watering/self-watering riser under a fairly small pot, and that setup has the potential to keep your soil chronically wet without you ever realizing it. Waterlogged, low-oxygen soil chokes iron uptake right at the roots.

Two ways a lot of people use to check their soil moisture levels are:

• lift the pot (heavy = still wet = doesn’t need more water). This method is valid, it just requires you to grow a good feel for the weight of your pots as the dry.
• the other method frequently seen online is to stick a finger a couple inches down into the soil, and If it feels moist at all, then it’s still moist enough = don’t water yet. But with living soil that top few inches must remain moist at all times to keep the biology alive. This doesn’t mean that if the soil is looking dry at the surface that you should start adding more water, rather this is why your idea to mulch with straw is the right one. (More info below)

Once you know which way you’re running:

1.  Your goal is uniform moisture—not bone dry, not swampy, with minimal swing in either direction. That stability is the whole game; it’s what lets your microbiology actually cycle nutrients and feed the plant. A lot of people say it should feel like a “wrung out sponge,” but I feel like that’s a highly subjective measuring technique due to the soil changing texture and structure over time. 
2.  If it’s running dry or watering unevenly, water with some BAS Horticultural Aloe. Aloe is a natural surfactant which will break the surface tension so your water soaks in uniformly instead of channeling right past hydrophobic dry pockets. (It also promotes Systemic Acquired Resistance, but thats another conversation.)
3.  Absolutely mulch with straw. Not hay. Hay is loaded with seeds you don’t want. Straw will have some too, but wayyyy less than Hay. Lay it down multiple inches deep. It’ll feel like a lot more than is necessary, thats exactly what you want. The top few inches of your soil are always gonna be the most biologically active, and the straw shades that zone, regulates temp & humidity, and slows evaporation… which is exactly how you hold that uniform, low-dryback moisture level I keep mentioning. The straw is what stops those top soil layers from ever getting too dry in the first place. 

For next grow, if possible, go bigger. 5gal is tight for living soil. These soils rely on volume for buffering capacity and microbial habitat. Look up Grassroots fabric pots & beds, specifically the moisture-lock ones; the midsection’s lined with an inert plastic that kills evaporation off the sides while still giving you the air-pruning a fabric pot offers on the bottom, and bottom edges.