I am now inching towards the Macap M4D.
I have had the M4D for 7 years now and have been very happy with it.
As to grind retention, it is minimal, no more than 2g at the most, and that depends on roast depth. Darker roasts that have more oil, tend to be retained more than light roasts in this grinder. Overall the retention is negligible, to be honest.
If you roast light and like to grind super fine, then I would not recommend a M4D, but something on the next level.
I have used a doserless Kony E, and it had retention of around 18-20g. too much mucking around for me, though the grind was excellent.
I was in the same boat as yourself, looking for a grinder with minimal grind retention, as I usually single dose.
There is an Electra Nino grinder for sale on CS, they want too much for it, but you may be able to negotiate a deal with them. The Nino has been around for years, and has minimal retention for a Conical.
For myself, I looked long and hard and have settled on an Etzinger EtzMAX as the best upgrade from an M4D. They are easy to clean, have almost no grind retention and are built to last. They are grind by weight and have a small footprint. Though tempted by all the Conicals that have been coming up, none suit my needs as they all suffer from grind retention, are too big or just don't take my fancy.
Good luck with your upgrade, let us know what you decide upon.