I want to convert a 1-piece crank triple chainring to a double
Yes, you need to consider chain line, but that does not mean you cant swap in larger rings on the middle and outer positions.

Sounds like right now you are are using the middle and outer chainrings exclusively on your triple crank. If you put two larger rings on the middle and outer positions, there's no actual difference in how you are treating the chain.

Obviously it would be better to use a double crank as this will put less strain and wear on the chain, but using the two outer triple rings may be acceptable, as it's the lowest cost option, especially if you are careful to always avoid cross-chaining from the large ring.

BTW, you'll want to make sure that there is clearance between the new large rings and the chainstay.