OK, so that's 36% on Z4 and above, mine is only 24%, I probably got the FTP too high and as a result the zones are not quite right... I'll put it down to 240 W, which is probably more in line with what I thought it was
I'm not convinced that riding the same and moving the FTP around is the right way to go about things.
Let's take a step back, and tailor your rides to what you are training for. Most hill climbs are between 2 and 7 minutes, right? That's VO2 territory, zone 6. It also gets you a great bang-for-your-buck in training, but it's easy to overdo it. (It is also an aerobic effort, so a big FTP is valuable for this sort of effort.)
The most common indoor VO2 rides are quite a deep square wave - minute on, minute off kind of effort where "on" is 120% of FTP, and "off" is 60% or so. Repeat a few times, have a 5 minute break at 60% and then do the whole set again. Do you have any small hills where you can just smash out 300W for a minute or more? It's easier to do longer intervals outside - doing a 30 second climb, turning around and descending and then repeating 10 times feels really daft. However, you say you have sharpish short bumps - if these are 30 seconds then that's what you have got. (Where are you if you don't have a 2 minute climb handy - I thought Oxford was flat but we have a few 5 minute jobs around here.)
You probably can't do more than a couple of these types of sessions a week without putting yourself in a hole (typically the TR build stuff does VO2 Tuesday, over-unders Thursday, sweetspot Sunday), so it's worth experimenting with a mixture of these (an hour or shorter to start with) and some threshold work (Zone 5) as well as ordinary rides of the format you posted.