i actually had this as a goal last year (pre-43things) too and my hairdresser solved it for me.
she recommended KMS california. colourvitality. suitable for tinted hair. i suppose there would be a version for natural hair as well.
It cost me about $21 each for shampers and cond.
I guess i bought it in about september and now its february. there’s still a fair bit in the bottle.