I don't think you are going to be able to buy an off-the-shelf ("we do it all for you") ADC chip that has 24-bit precision. Note that accuracy != precision != resolution. Your ADC is not going to be more accurate than the reference. At best, this parameter is limited to somewhere around 1ppm/year. The precision is the INL spec; this means that each bit has meaning, and is repeatable (and/or predictable). You can have 1000 bits of resolution, but if those bits are not repeatable and/or predictable (and linear) from measurement to measurement, then they are a waste of time and money, and you are just fooling yourself. Noise can only be filtered out if it is Gaussian. For 7.5-digits, what you are looking for is >= 25-bits of precision (which is the INL spec, even if you need "help" from the firmware to achieve that).
You *CAN* get the necessary precision for 7.5-digits (or even 8.5 digits, but that is difficult for a hobbyist), by using discrete parts (precision op-amps, low-noise dual FETs, high stability resistor ratio dividers, etc.) Probably, the best architecture (these days) is "Delta-Sigma", but implemented with high performance parts, using multiple integrators and a multi-bit feedback DAC-- but not with a monolithic IC.
Probably, a good "first try" would be a 6.5-digit ADC (with < 1-LSB INL)-- if you can get to that point, then you will know what needs to be improved to get to 7.5-digits (with < 1-LSB INL). Going from 6.5-digits to 7.5-digits will take at least an order of magnitude of effort, and maybe even an order of magnitude of cost. 8.5-digits (with <= 1-LSB INL) would (probably) require some custom parts as well as some highly selected parts in order for the last digit to have any meaning (other than as a marketing tool). Going from 7.5-digits to 8.5-digits would require another order of magnitude of effort and BOM cost. Also, at 8.5-digits, you are getting into the realm of Black Magic and Fairy Dust. That's why I said that 8.5-digits is probably not something an average hobbyist can achieve. I'm pretty good with designing analog electronics, but I don't think that even MY "Analog Fu" is powerful enough to pull off a good 8.5-digit ADC design all by myself.
There is a possibility of working on a "community designed" 8.5-digit ADC (with DMM front end)-- if the top talented engineers here on this forum would work together, then this might just be achievable. It would certainly be a lofty goal to strive for. I've noticed that there are a lot of knowledgeable engineers here, and that is a lot of "Engineering Fu" in one place-- maybe even more than someplace like Keysight or Fluke. The problem is, getting 3-dozen (or so) highly-talented engineers (usually with big egos) to work together is a lot like "herding cats"; Good Luck with that!