I Have Alpha 4500 6 speed on one of my bikes a older MTB and have no real issues shifting with the original AR freewheel and have got good results using spare wheels with 7 or 6 speed Shimmano. Alot of shifting good with Suntour accushift has to do with shift skill ability you need to hold push light on the index shift till the gears are shifted then let go and make smart reasonable shift choices which is easy for for riders having road a lot on just OK friction where this is pretty basic. But is a a learned skill for riders that have used mostly nice indexing modern better 7+ speed Shimano and Sram stuff. One thing that may help that I have found is to get cheap derailluer frame extension about $5 online and it will give the bike a bit more modern DR mount for the modern parts.
https://www.ebay.com/itm/Alloy-Bike-...53.m2749.l2649
Last edited by zukahn1; 07-09-20 at 07:07 PM.