Alvin, others will have better answers, but I was in exactly your position a year ago -- a late upgrader with quite a few plugin dependencies worked into about 14 years'-worth of TiddlyWiki Classic projects. (Nested Sliders, and the combination of aliases and Redirect Macro, were core to my workflow.)
I couldn't find anything that converted my existing tiddlers over to an equivalent TW5 function; that is, all the +++ and === syntax in TWClassic had to be reworked more or less manually, if I recall correctly.
Also, I should warn you that the word "Slider" has gotten used for multiple functions, so searching for it leads down some dead-ends.
It's not nearly as quick to deploy on the fly (in terms of typing out markdown like +++ and ===), but the tradeoff is that it's more powerful, at least for my purposes.
I'll post this for now, in case you want to check that out. Meanwhile, others might chime in with what works for them, and I'll also keep an eye out for follow-up questions.
-Springer