Well done on germinating the seedlings, I must have a go..I,d try Baby Bio feed at present Tomato feed will encourage flowers but you want strong growth first.
I have a Lemon Tree ,it's about 4 years since I bought it and provided I don't drink too quickly it supplies me with lemons...about one every couple of months.
It lives in a cool conservatory over winter (popped outside on bright sunny days) and outside from April to first frosts.
Watered with rain water (our tap water is very hard) and fed Citrus food when living outdoors, not quite as glossy leaved as others seen in shops and nurseries but healthy and the scent of the flowers is amazing.
I buy Citrus Food from garden centre or RHS Wisley,I live about an hours drive away ...really any excuse to visit.