@ChrisCycle What you are describing is what is know as block training. There’s a separate thread on VO2max block training if you are interested (link below).
Sweet Spot block training is a very effective way to maintain a high level of general fitness or prepare for B race/event (at the suggestion of @KorbenDallas I did it prior to a recent 40mi/4kft road race I did recently and it served me well).
Threshold and over/under lactate clearing is interesting, but you will accumulate substantially less Time in Zone (TiZ) than sweet spot, so I am not sure if it would be incrementally as effective as SS training or what purpose it would serve on its own.
For a deeper understanding of the value of training in each zone, you may want to read chapter 5 of of the recently released 3rd edition of Allen/Coggan/McGregror’s book, Training and Racing with a Power Meter.