Honestly, the Kaisi is kind of overkill and unnecessary. The connectors are on the front, and only takes a second to swap. Really, you have a good set up. Both Aixuns have really good hibernate functions, and if you're limited on bench space, they'll stack on top of each other. The T3A and T3B together are cheaper than the 420D (and again, takes up less space), plus I'm a little hesitant about the 420D in that both irons share one screen and PSU. If something goes wrong, you lose both sides. I think the only thing you MAYBE lost out on cost would be the t210 handle for the t115, and even then, there are times when I could see you using t245 and t210 at the same time to remove IC's, multi pin connectors, etc. It's a good thing to have 2 soldering irons, and the t115 just doesn't have the "oomph". Instead of getting the Kaisi, I'd look more into a preheater solution, and probably a hot air station. Then there's literally nothing you couldn't do soldering-wise.
For hot air, the Quick line off of Aliexpress is the most budget-friendly. But reliability can be an issue. Screens burning out seems to be the most common. The Quick 861dw performs really well, and is probably their best unit. Maybe a bit expensive though, so if something goes wrong... Just don't get a station that is basically a hair dryer connected by a power cable to the base unit. You want the hot air coming out of the base itself.
For preheaters, the Sainsmart/Miniware MHP30 is a functional (if not powerful) little device. Reasonable priced if you get them off of Aliexpress. But they can have issues too. Seems like the heating diodes can burn out (and I didn't see any way to replace them), and the temperature is somewhat off. It's not for heavy jobs, just spot heating. A full sized hot plate or air heater is large, and depending on what you want to do, is just entirely too much. Not to mention an air heater is just uncomfortable to be around. You get preheated too. If you really want to build something, there's a good youtube vid on a DIY preheater that looks really cool. I haven't made one yet, but it's on my list. He makes mention of separating the unit into 2 parts- the control unit, and the heating plate, so the heater is disposable. Which is totally do-able. Good luck!