The standard 6502 assembly is already "weird" in that some operands are rolled into the opcode, but some are arguments.

E.g. various register-register transfer combinations have dedicated mnemonics: TXA, TAX, etc.

LDA (index),Y. has index and Y in the argument space, but the A operand is in the mnemonic. The comma doesn't separate operands but is a displacement operator.

In other words, we really want LD A, (index) + y. Or LD A, (index + x).