Carrots came out forked and stubby from a raised bed I filled last spring Soil & Compost

Bagged topsoil is the other half of the problem. It is usually screened but still full of small stones and clay clods that break the taproot. I ran mine through a 10mm sieve for one bed as an experiment and the difference in straight roots was ridiculous.

Each shape tells you something different. Forking means the tip hit an obstruction or a lump of undecomposed compost and split around it. Stubby with a blunt end means it hit compacted soil or the bottom of the bed, so 30cm is borderline for a full Nantes and definitely too short for anything longer. Hairy with side roots is nitrogen, usually from fresh compost or that 10-10-10 you scattered. Carrots want lean, loose, evenly moist soil and no fertiliser at sowing.

Also check watering consistency. Dry then flooded gives you split roots and blunt tips even in perfect soil. Carrots are boring like that, they want the same moisture all the way through.

Forked carrots taste identical and store fine, for what it is worth. I gave up chasing the perfect straight root and now grow twice as many in half the fuss.