Why choose just one?!
When harvesting for multi-purpose hemp, generally, it will be a combination of CBD production, combined with one or both of the other desirables (seed/fiber). Your planting, and therefore, your harvesting method will be entirely strain dependent. You may consider wide spacing like in CBD Isolate/Extract farming, or it may be easier for you to do close spacing with high density. When farming multi-purpose hemp, the choice is truly yours, based upon the equipment you already have and the level of labor you are willing to expend.
When farming for multi-purpose hemp, separating your materials cleanly and efficiently is the most important step in the harvesting process. You may want to consider this when selecting which planting/harvesting technique to use.
The harvesting methods for these plants are all over the board. Farmers can use any of the methods previously described. It really depends on which of the multi-uses is most important to you as to which harvesting method you choose. If you care most about your extract/isolate, then you may want to consider the dry and cure method. Adversely, if you are more focused on the seed/grain material collection, you may want to just consider the traditional combine/wet roll into bail method.