I am trying to make a cheaper (500ish) bike work with a Tacx Flux (2018 version). According to their website, the Tacx Flux only supports 8-11 speed cassettes, but my bike has a 7 speed freewheel (14-28T) on it. So I looked up and found out that I am able to use a 4.5 spacer to fit the 7 speed cassette on the Flux's cassette hub. However, all the 7 speed cassettes that I found online have 11 or 12 teeth top cogs, while the derailleur on my bike requires a 14 teeth top cog. I was thinking of just replacing the top cog with a 14t one, but I couldn't find a way to just replace one cog in a cassette. I am new to this stuff and wasn't sure if my understanding of the situation was correct. The Shimano website states that the top sprocket max and min for my derailleur is 14T (https://bike.shimano.com/en-EU/product/component/tourney/RD-TY200-GS.html) Does this mean that I have to use a 14-28T cassette for my bike to work or am I understanding this incorrectly?
Thank you in advance!