How are you supposed to clean a sump anyway? The reason I am not so sure about a sump is because I'm not very handy and really wouldn't trust something I made. I know someone that could build and ship a 75 gallon sump to me for $1300. The idea would be filling it 1/3 of the way up to make room for more water in case the power goes out. Making it myself I guess would cost me $600. Two FX6's cost about $1000 all together.
What?! If that is how much custom sumps go for, I need to get into that! Is that a typo, or did you mean $130? I got a RSR, so the sump came with it. I know you said you weren't that handy, but used 75g tanks are dirt cheap and you could watch some youtube videos to get a feel for how hard it is. Also, I see a bunch of sumps on Facebook Marketplace local to me that are cheap. Maybe that is one of those fancy ones with different colors and all that.
If you have the overflow right, you shouldn't need that much room for a power outage, I don't think. As long as you have syphon breaks in the right places. I just got a VFG for my tank and I need to drill a small hole for that cause I noticed it likes to pick up syphon lower than the weir.
As for two FX6s. I have one I bought used. I see them all the time for between 150 and 200 on Marketplace. The impeller is 60 bucks I think. I have one as a spare, although mine works great. Since moving to the sump, I wouldn't go back. I loved the effectiveness of the FX6, but unhooking it and taking it outside to clean was just enough of a pain to make me put it off until the microbubbles got on my nerves.
The sump, I just reach in and slide out the filter socks/cups. Heck, with the cups, I just grab the top layer of poly-fill and pull it out. The socks I just swap out for clean and then clean the spares at my leisure. Now, I haven't had to do a thorough cleaning of the sump yet though, and that will be a much bigger pain.
HTH in some way.