I too support previous comments and your own statements about having a vision for your photography. That is most important and could very likely change as you branch into new areas of exploration/creativity in this wonderful avocation/art we call photography.
I shoot 35mm, MF and LF and I love all three formats for vastly different reasons. For 25 years, I shot 35mm exclusively and given the amount of travel I was doing, it worked well for my needs. Then, about 5 years ago, I discovered the wonder of LF. It was such a radical departure from the travel, journalistic, and sports photography I had done and I purchased, without hesitation, a 4x5 monorail and have not looked back. There were so many new avenues to pursue in technique, equipment, composition, subject, and of course, printing. I was thankful for the emergence of digital too as it had served to depress LF prices on equipment. Sweet! That monorail and lens purchase eventually migrated to a field camera and several other lenses, as well as medium format film backs when I did not want to shoot 4x5. That capability provided considerable flexibility with little weight penalty (especially when travelling with the 4x5).
One of the ongoing issues I had, in my printing, was obtaining acceptably sharp 11x14 prints from 35mm. Well, with the 4x5 negatives, those issues were a thing of the past.
About 2 years ago, the advent of the digital frenzy had a highly inviting impact of severely depressing the prices of MF equipment. I bit and purchased an RB67 with a lens and a couple of film backs. The 6x7 format provides excellent negatives with very good sharpness retention through larger print sizes. What started as an basic outfit since grew to a half dozen lenses (often at only $1 per mm focal length), and additional filmbacks and other accessories often obtained for 10 cents on the new dollar.
Of course, there is usually some carping about the weight of the rb67, but you will not find a more rugged, reliable instrument. And, the weight issue is overblown. I have a setup where I have a body, 6 lenses, and other assorted accessories in a Lowe Pro Nature Trekker AW. Yes, it has a bit of heft, but it is not that onerous.
With respect to a 645 camera - I did not consider it at all as the negative format is more like 35mm on steroids. If I want to shoot 645, or even 6x6, it is just a matter of getting film backs in those formats for the RB67.
In the end, I did not plan to get into MF at all, but the prices on equipment were too inviting. At times when I do not want to shoot 4x5, or on very long range trips, the MF provides that capability of excellent negative quality and relative portability.
The 4x5 remains the bell weather of creative landscape and macro/still life work, and the 35mm equipment is pretty much in the sports and wildlife subjects.
I use all of the equipment and have an enlarger to print in all formats. For me, it just comes down to deciding what I want from a particular subject and applying the equipment and technique of any of these formats that will give me that result.
Have fun with your deliberations. Whatever you decide, as long as you enjoy doing it, you have made the correct choice.