Is it relatively simple to design an ADC to ADAT interface with only a Toslink port?
Those two chips are a good choice and would do the job, but as for being “relatively simple”? No, and certainly not for a “relative beginner in electrical engineering”. If nothing else, you’d have to make a fairly intricate surface mount PCB with not only these two chips but all the peripheral stuff: power supplies, clocks, audio preamps (mic? line? hi-Z?) And then put it in a box and power it from somewhere.
But assumingly you could do all that, you’d want to generate your own 44.1KHz or 48KHz clock domain (word, bit, and master clocks), configure the ADC in master mode, and route the four i2s outputs into the ADAT transmitter (configured to use 24 bit format). The Apollo Twin would then need to be set to use your incoming ADAT as its clock source.
I commend you for finding good chips for doing this, but I would gently dissuade you from undertaking this rather audacious project unless you really want to do it for fun and learning. Otherwise, go to Sweetwater and plunk down $400 for the Focusrite OctoPre.