Quoting OldNavyRefYour job as referee is to apply law and competition rules, not what you feel they should be. Starting to adjust rules to fit what you feel they should be is a slippery slope - even where this example is relatively harmless (and will make your life more difficult rather than other officials), it's the start of the path that leads to statements like "I don't do sin bins" and other LWR behaviour.
I'd also caution you against getting correlation and causation mixed up. An assistant may be biased and an assistant may be coaching from the line. Those are two unconnected actions - a biased assistant should of course be removed, but assuming that coaching means they will therefore make biased decisions is an unfair logical leap.
My only real experience with this was quite the opposite - I recall one assistant who was definitely overdoing his coaching. But when the ball was played through and the opponents scored, he kept the flag down and shouted to the defence "I told you that you needed to be a yard higher up!". Coaching to me implies someone who is paying attention, which is infinitely better than the type of CAR who just wandered around behind play and sticks the flag up without thinking.
"Coaching outside the technical area is not allowed" Law 1.9 does add weight to this. 'only one person at a time is authorised to convey tactical instructions from the technical area'. But in the opening paragraph of 1.9 it says this applies to grounds with a technical area 'The technical area relates to matches played in stadiums with a designated sitting area for team officials, substitutes and substituted players as outlined below'. Which most grassroot grounds don't. I think for me, when paired with CARs, this falls in to the 'Spirit of the Game' area.