they seem to be fairly hungry trees. and especially like lots of organics.
lots of compost and mulch definitely help.
iron, magnesium and/or nitrogen are usually the basis for chlorotic leaves.
watch PH, they like acidic soils.
a little Epsom salt can help. it has magnesium and the sulfur lowers the PH.
dont use too much, it can lockout calcium, but epsom also washes through quickly.
1 tsp per small tree 3 times per year (soil test is best to see magnesium levels though)
i use a citrus fertilizer at 1/2 strength but also compost, and fish emulsion.
Carambola trees generally develop iron, magnesium, and manganese deficiencies when grown in soils with a pH above 7. Symptoms of iron deficiency are interveinal chlorosis (green veins with yellowing in between), reduced leaflet size, and, with severe deficiency, leaflets may become almost white.
https://edis.ifas.ufl.edu/mg269#:~:text=Carambola%20trees%20generally%20develop%20iron,leaflets%20may%20become%20almost%20white.Soils: The carambola is not too particular as to soil, but will grow faster and bear more heavily in rich loam. It prefers a moderately acid soil (pH 5.5 - 6.5) and is sensitive to waterlogging. The plant often becomes chlorotic in alkaline soils.
Irrigation: The carambola need moisture for best performance. This means regular watering during the summer months and must be watered even in winter during dry spells.
Fertilization: In soils of low fertility young trees should receive light applications every 60 to 90 days until well established. Thereafter, they should receive one or two applications a year in deep soils or three or more applications in shallow soils where nutrients are lost by leaching. Application at the rate of 2 lbs per year for every inch of trunk diameter is suggested. Fertilizer mixtures containing 6-8% nitrogen, 2-4% available phosphoric acid, 6-8% potash and 3-4% magnesium are satisfactory. In the more fertile soils of California, this program can be reduced. The tree is prone to chlorosis in many western soils but responds to soil and foliar application of chelated iron and other micronutrients.
https://www.crfg.org/pubs/ff/carambola.html