I would be curious from you, as well as other "above 300" guys the following approximations:
You have asked some good questions so here is my input on this complex subject.
1] What percentage of your total DXCC were contest QSOs
Contesting is a good way to quickly add to your totals when you are a budding DXer. I'd say there are something like 150 common entities that you can work and easily confirm using LoTW so its a great starter strategy. If you are chasing DXCC on multiple bands it's also a great way to add bands. It really helps on the low bands like 80m and to a lesser degree on bands like 160. Sometimes semi-rare or rare countries show up in contests. These can be good or bad depending on how good the operators are and if there are on in advance to the contest. But once you are above 200. I think that working rare new ones during a contest only happens a few times a year.
2] What percentage were arranged skeds?
Very few. When I was starting out, I thought that this might be a good strategy, but in general, stations that are active can be worked given time and good enough conditions. I have been known to beg on the air and via email or forums for a station to try 160m. For example J8 has not been active on 160m for almost 10 years. If a halfway decent station came on the air it would be pretty easy. But so far this strategy has not produced much reward. On the other hand, I did email KH2/N2NL when I heard him on the air and bugged him to be active on 160 as I needed Guam. As it turns out Dave did not need much bugging as he is a topband addict. When the conditions were right one sunrise, I worked him. It was not a sked per se. I was just at the radio almost every morning hoping for conditions for 3 months before it worked out. He heard me and actually worked me ahead of a topband big hammer as he knew I would likely fade if he did not. Dave posts here quite a bit and he is a super A1 DXer.
3] What percentage were you CQing and had the DX respond blindly to you?
A small handful have been worked as response to CQ. Some of them were ATNO but not that rare and would eventually be worked another way. But it is fun to call CQ and work even common entities. From the east coast NA working Asia requires better than average conditions for small stations like mine. It is exciting to be working JA and have a HL or BV, 9V or other rarer entity call you. It also is a good way to learn how a band works so you can be on the right band at a good time when it is really important for a super rare ATNO. So I would say it is not likely to give immediate payback, but is good DX training. Working 50W and wire antenna weak signals is also good ear training that will help in DXing.
4] What percentage were following cluster spots versus spinning the VFO and being first on frequency?
I like to turn the big knob and find my own stations. I don't know that this is the best strategy to add to your entity total. But finding the station by my own ears even if it's not rare is really fun for me. I spend a lot of time tuning the bands. I would not say it has added that many really rare ATNO. On the other hand I there are times doing this that you are early on the scene and can avoid the chaos that happens after a cluster spot. I'd say way less than 10% of ATNO have been caught this way. Probably sub 5%. But it's fun so I will keep on turning the big knob.
When it comes down to chasing something that is rare you need to pull out all the stops if you are a peanut whistle station. I don't mean bending the rules or acting like a fool. I mean do your research. Know when the expedition will start and end. Know what frequencies they operate and observe when they are on which bands and which modes. Pay attention to when you can hear them and when you can't. Do some data mining. I won't reveal any secrets here, but the cluster and other sources of data are not only for real time. Talk to other operators in your area and share info even if it's from earlier operations to the same entity.
If you know that there is likely a strange opening on a certain band and it's likely to happen in the middle of the night, make a plan to be there. The crowd will be smaller. Also I hate giving up this one as it is really powerful, but you seem like a nice guy. WARC band focus. Make or buy a good antenna. It's an equalizer. You have to hear the station to work them. It seems trite, but it's true. I live in suburbia and every year there is more noise probably this is a weakness for many smaller stations nowadays.
Don't wait until the last moment as many expeditions wind down toward the end and you might not catch the train. Sometimes you have to wait for the big dukes to pummel each other a bit, but don't procrastinate as there may be an emergency and the dxpedition will end early. So armed with your research you may be at the radio when something breaks and you might even get under the first wave. So I won't say these catches come from only turning the big knob and doing your research, but the whole thing planning and scheming helps sometimes...It is kind of combo thing.
I'm a real little pistol mired in the 270s with an HOA, and would maybe like to try some new strategies.
I think that the takeaway from what I have been blabbing about here is the more time you can spend at the radio the better your result will be. I am not saying to be addicted to the radio. Everyone needs to balance the rest of their life. If you just read other peoples advice it will not become as useful a tool as things you discover yourself even if you are not the first one to discover them.