I like twist shifters in general, and in particular, the old school SRAM SRTs and the higher end Shimano Revoshifters (SL-RS45 and above is a different beast than RS35 and below). However, there are indeed some notably poor examples out there as well. I imagine some especially cheap ones are made in ways such that twist motions to shift cause all sorts of unnecessary friction inside the unit, making them more difficult to turn. I have experienced these myself from time to time, especially on donate bikes. One thing to check here is ensuring the rubber grip is not jammed up tight against the twisting grip, causing friction. Name brand twist shifters have various ways of mitigating that, but knock-offs might value engineer that sort of thing out.
And there do exist some indexing thumb options, if you want to go that route. Look at Shimano's TZ500-6R shifter. It's a low cost indexing thumb shifter that should work with a six speed freewheel. I have the 7R version shifting a seven speed cassette on an older bike and it works perfectly. The shift effort seems about right and it just works well. It's an option if you don't want to go full friction.