I also use a Ulysse MM and it is roughly A5 size. I use the Hermès inserts because I like the paper and it's one of the better papers for fountain pens but it is easy enough to attach the plastic connector to other notebooks. I don't generally clip pens in the notebook spiral because I don't want to scratch my pens or damage the clips but it does work well with less expensive pens which I don't care whether they get scratched or their clips get deformed over time.
The Neo version includes slip pockets on the front cover which can come in handy at the cost of weight, the front cover edges not matching the back cover edges because of the stitching on the front cover. I'm not a fan of the look of that stitching on Togo covers. I like the clean stitch-less look of the original Ulysse.
Another intriguing option is the EAZip:
https://www.hermes.com/ca/en/product/eazip-book-cover-H074743CK7L/
Unfortunately, it does not have a snap closure that you are looking for but the pouch looks handy and it's designed as a cover for any standard A5 notebook.
There is also the Writing Set:
https://www.hermes.com/us/en/product/writing-set-medium-model-H068992CK45/
This one is extra luxurious but not easy to find. It fits A5 notepads. I think you can fit your own but the Hermès ones are slimmer so it won't feel as bulky and you can fit two Hermès ones. They are also made of Box so they will scratch up (which is part of Box's charm) but not everyone likes that look.
I hope this helps.