I have dealt with a bunch of M-ATX boards with plenty of access front and back. That shouldn't be too hard to find just in a search on Newegg for something you like with those features, such as cutout in the motherboard tray so you can get m.2 mounted on the back of mobo, etc.
M-ITX, now that is a whole other story. I have owned quite a few. I think the Cooler Master Elite (I cannot recall the model #) was super easy to build in and access for most things. It was almost completely mesh so the biggest thing I didn't like was with little to no space for wire management you just sort of had to bundle them up and ignore it. Thermaltake makes a small tower M-ITX system that I have built in and it's pretty easy to deal with.
I think the best advice I could give you on the M-ITX is to purchase an SFX power supply with adapter plate and use a fully modular PSU so you can only use the cabling you need. Plan out your build as you select parts such that you know things are and will go where you want them.