DS turns 7 in a couple months and we're also still stuck in limbo. Last year we gave up trying to get him to ride his 14" bike with training wheels, and so the past summer he had no bike at all because he was way too big for the 14". I kept meaning to get a 16", but I waiting too long and now the season is over. We did get him this Razor trike: https://www.creativekidstuff.org/store/ck/item/58294/razor-flashrider-360-caster-trike?source=4-3128-19
It's a cool concept (easy to ride, it's on castors so you can flip around easily, and it shoots sparks when you pull the break- it also looks "cooler" than your typical trike). DS still wasn't impressed. We used the tagalong most of the summer.
I know we need to get on the bike situation soon, and I'm trying to talk DH into this bike: https://www.rei.com/product/851112/electra-mini-rod-1-16-bike-2014
DS saw it at REI and hopped on it immediately. The seat sits lower to the ground so it's supposed to be easier to learn how to ride. It's more like a "cruiser."
I haven't been on this board in forever, but I hope you don't mind me giving advice.
My DS is now 5 and started on a balance bike. I'm not sure if they make ones that are bigger for bigger kids, but you can look into it. It has NO pedals at all. When people would see him in our neighborhood, they never noticed that he was using his feet and it didn't have pedals.
If you don't think that would work, I would try to encourage both. When my DS started on a two wheel bike, he still wanted to ride his balance bike everywhere since it was faster for him. However, we did make it practice on the other. Once he got faster on his pedal bike, he transitioned over. It would be hard to give up the "fast" ride for something that it slower for you.
I hope I helped a little bit!!!