You can setup the value to 5 kHz directly before the first 1kHz appearance, yes. Then the complete song before the apperance will act as 5kHz, as long as you are in line mode, and the patterns before first appearance have no automation. So the very first value and also the very last value will be kind of “inherited” to the pattern before/behind.
There is also a difference, if you enable “automation follow” in the song settings. This causes the inherited value to be updated as soon as you scroll, use aliases or jump in the song. If it is disabled, only at song start, the inherited value will be caluclated once (or something ). I guess there is a section in manual, please have a read.
There were some problems with automation following and live recording parameters thru VST GUIs and/or midi. Maybe it was fixed, I don’t know. For know, just leave it checked.
Using two filters does not seem to bring any benefit here, because then you have to manually automate their on/off states, and on/off automation is a bit “retarded” in Renoise, it only works as pattern commands, will easily turn into other automation, if you accidently do not use enough fx columns or multiple on/offs…
I highly recommend to you to automate devices only using the drawable automation.
As for finding the last set automation point, yes, this is cumbersome. There is no “jump to last automation value/pattern” functionality AFAIK. Maybe in a tool, I don’t remember.
You also seem to be able to automate on/off state thru a meta device, so you could use a free draw LFO, which flows into a hydra device, which flows into the on/off states of your two filters. Then you could trigger “reset” of the LFO, to have a range where the other filter is active, or similar.
Hope this helps.