A long reply because there’s a lot to cover, the races you’d like to attend with the classes you’re looking into all have their ups and downs:
Canaan (NHKA): If you’re starting to get bored running World Formula at Canaan, you will definitely be bored running the slower 206 engine there. Especially since the 206 classes are smaller than WF in the NHKA.
Rock Island and Elkhart: There was no TaG class at Rock Island this year. There was no TaG class at the Elkhart GP this year either. That doesn’t necessarily mean there won’t be a TaG class at either event next season, but don’t be surprised if there isn’t.
However, there was a TaG Masters class (Age 30 or 35+, dependent on weight) and a IAME-only TaG class (16+) at the 2018 Brickyard race.
Daytona: They have a licensing system for the higher-hp classes like shifters for their Road Racing events. Recently, I was going over the minutes of one of the WKA Directors’ meetings, and it looks like they’ve made the experience levels needed to get a license to run a shifter at the Road Racing events more stringent. Your World Formula experience at Canaan might be enough to qualify to run a 125 shifter at Daytona, but I don’t know for sure. You’d have to contact the WKA to see if you’re eligible.
One thing I know for certain, you are definitely eligible, right now, to run a TaG kart at a WKA Road Race like Daytona. WKA has a sizable “IAME Sprint” class in Road Racing.
So there’s no perfect class to hit all of the races you’re looking to attend, you’d have to consider the advantages and disadvantages of each engine on your own.
Two other things to consider about racing a shifter kart: there is more “stuff” to maintain on a shifter, and most gearbox karts do not have an electric start like TaG has. Have you talked to Dave Nadeau about his experience with a shifter? I know he dabbled with a Stock Honda while running World Formula in the NHKA. Keith Buffo is another driver who has done both.
Another idea, if you really want to bang gears, see if you can arrange an Arrive and Drive with one of the local shops like DRT, even though it’s late in the season, to get an idea what it’s like to drive a shifter kart, before you jump in.
One thing you can possibly do, since you already have a chassis for it, is get a TaG engine, and rent a 206 motor for Rock Island and Elkhart (big 206 fields there), I wouldn’t be surprised some shop or team would have 206 engines available for those races.
Finally KA100; it’s a really attractive package, but performance-wise, it’s right in the middle of WF and TaG, and I just don’t see the KA100 gaining a foothold in the NHKA or X1 while the World Formula is as popular as it is now, (and still growing). There just isn’t incentive for all of the WF racers right now to make the jump from such a healthy karting class to start a new one.