There are a few bike frame details you need to think about for your bikepacker. 1st: The frame differences between a "gravel bike" bikepacking frame and a "29er hardtail" frame used for bikepacking. Gravel frames have a shorter wheelbase, steeper headtube angle and lower bottom bracket. 29er hardtail frames have higher bottom brackets, slacker headtube angles and a longer wheelbase. 29er frames tend to weigh more. Most 29er frames these days use boost 148 rear hubs and 110 front hubs. Gravel frames typically use 142 rear hubs, and 100mm front hubs. 29er frames usually handle up to 2.3" wide tires, gravel frames, 48 to 50mm tires. (On Aliexpress, there are a lot of 700C based gravel forks that have mounts on the forks, and only a few 29er carbon forks that have fork mounts.) 29er forks tend to weigh about 100 grams more than a gravel bike carbon fork.
With the above written, ask yourself if your "Bikepacker" is going to be used primarily for racing or for touring. For racing, you will want to get the lightest weigh bike you can build. For touring, weight is not as important, but comfort and capacity you can carry becomes more important.
Regarding frame mounts. Frame mount don't add much weight, so I believe in having the versatility of being able to put on racks and other accessories if you eventually decide you want a rack for a specific ride/trip.
One other feature to consider. If you are going with a 29er frame, try to find one with a larger triangle area. This will let you carry more gear in a triangle frame bag.
To put this all in context: I had a custom Waltly TI frame built for my 29er bikepacking rig. I have a very small frame triangle and can't carry much gear in the frame triangle. I wanted to use only a seat pack, but I am addicted to dropper posts, so with a seat based pack bag it hits my rear tire when my dropper is down. Therefore I ended up installing a rack and run small panniers and a bag strapped to the rack.
My 29er bike packing rig is so much more comfortable than my gravel bike, but due to all the weight I often carry, the bike is a lot slower riding overall.
