Your middle image (despite the caption) seems to show the center boom block. It is hung from a boom bail. You could rivet a similar bail at the end of the boom or through-bolt a U-bail like the one I posted earlier. Either one would work.
If you use a ratchet block instead of your center boom block, then you don't need to cleat the main sheet and can hold it in your hand. I use a Ronstan 55mm Orbit ratchet block set to auto ratchet. In a recent review in
Practical Boat Owner it was shown to have double the holding power of other blocks for the same purpose. More holding power means that the pull on the mainsheet from the sail has to be double to overcome the force you apply on the working end of the sheet. Because the ratchet feature engages automatically, your sheet will run free whenever you don't provide tension on the working end. One downside of the strong holding power is that the block will chafe the sheet. While the effect is visible, it's not something that worries me - it will take several more seasons before I need to swap my main end-over-end because of that, and even longer for a full replacement.
The key point is that with a ratchet block you separate the effort of trimming (making an adjustment, short term pull) from the effort of holding (long term pull). I find that I cleat the main a lot less with the ratchet block and that holding it in my hand for handling gusts in higher winds is feasible and no longer as tiring.
One upgrade I would make is to switch your swivel cleat so it releases
up, not down. I still use the original swivel block that came with my DS and it releases by pulling up. It is not any harder to cleat it, but definitely much easier to uncleat it. With other swivels I've found myself "trapped" with not being able to position my hands in a way that I could pull the main out of the cleat.
Now, let's discuss purchase.
First, if you pull in the middle of the boom you will need to pull twice as hard for the same effect as when you pull at the end of the boom. If you currently have a 4:1 center boom purchase, that's equivalent to a 2:1 end boom purchase. A 6:1 at the center would be the same as 3:1 at the end of the boom.
Mine is a 2.5:1 at the end or 5:1 center equivalent. There's a 2:1 purchase at the end of the boom, and then, the mainsheet when it comes from the middle of the boom to the CB trunk swivel block again pulls with the same force downward, but this time at the center (or half the lever arm). Hence 2.5:1 -- or 5:1 center boom equivalent.
If, as you report, your current setup is 3:1 at the center, then switching to the layout that I describe would gain you in overall purchase. Add a ratchet block into the mix, and you'll be surprised how much more "user-friendly" the system becomes.
In terms of "moving up to the cuddy" -- for fore-aft balance you want to sit rather further forward in the DS than you might think at first. It's really tempting to sail the DS while sitting far in the back, but when you do that you submerge your transom, dragging it through the water with the effect of increased drag. (Sailing with the parking brake on). If you had a mid-cockpit traveler, then when single handing you'd sit forward of it.
A 4:1 vang would be useful in controlling the height of the boom when reaching and running. For upwind, if you have an aft traveler (bridle), that may be sufficient. It depends on your style of sailing. Upgrading a vang after you've gained some experience with a 4:1 system might be a good route to go. That way, if you are happy, you can forgo the upgrade.
Finally, a topping lift. As the topping lift is usually loose while under way you can wrap the end around the horn cleat you have at the end of the boom when you need it. (In really light winds a topping lift that can be left engaged might give some advantages, but I wouldn't let that consideration drive the setup unless it's a common occurrence where you sail).