Yes, I think this is the only way currently to handle time slowing within a Fusion Clip. The Fusion Clip on the Edit page cannot be extended beyond the length of the media within it. And the Fusion comp on the Fusion Clip (in the enclosing timeline - eg "Timeline 1") cannot be longer than the length of the clip.
Your workaround of adding some junk extra clip is a valid way of extending the length to whatever you need. I would put this on a separate layer, eg Layer 2 if you're currently only using Layer 1, so the junk clip (eg generator) appears in a separate MediaIn node, and cannot affect what you are doing with the main MediaIn.
If only one clip was involved - ie a Fusion Clip was not necessary, because you only need to access one clip in Fusion - then the natural solution would seem to be to use an Adjustment Clip above the clip involved on the same timeline. Adjustment Clips with Fusion compositions on them access everything below them as MediaIn1, and can run for any duration on the timeline.
However when I tried this the other day,
I found some serious problems with using an Adjustment Clip composition with time-adjusting nodes like TimeSpeed and TimeStretcher. In short, it just didn't work at all. I need to re-test that to be doubly sure, then I'll raise it as a bug (it may well be known already).
In the meantime, your workaround is I think the correct way to do it - messy as it is.
This is a fundamental issue with the way Fusion compositions are currently handled in Resolve. They are quite inflexible and have a lot of rough edges and little bugs and issues. Hopefully this will improve in future.
My personal wish is that one day all Fusion compositions in Resolve will be flexible media items that can be placed and adjusted anywhere on any timeline, with their lengths set by the user irrespective of the media involved. In other words, I would like it to be possible to place a clip like the current Fusion Composition, which can be placed anywhere and set to any size, but for it also be possible for this composition to access a clip from a timeline (like a current single-clip composition), or multiple clips at once (like with a Fusion Clip) or multiple clips sequentially (like with an Adjustment Clip).
Such a change would make it much easier to understand where Fusion compositions are and to manipulate them, and would allow for situations like this where the composition needs to be a different length to the clip(s) it uses so time adjustments can be made. It would also allow for easy copying of compositions between clips, and ideally also allow a single composition to be used in multiple places, with different clips, without each copy requiring separate updating.
Fingers crossed that v17 brings some improvements in this area.