yea sitting most of the day is tough, hard to offset with any fitness/training routine, a little hack i seen via the huberman lab podcast was that doing calve raises (while sitting) has some wild overall fitness benefit. if doing the supplement thing, checking out the app cronometer, an app that tracks both the macro & micro nutrients of your diet such that you can get something of a gauge of what you might need/want to supplement such that one is able be more targeted rather than taking vitamins that aren't actually necessary
stretching (passive & dynamic, i.e. stretch & holding vs. no holding stretch but bouncing in & out range of motion) is one of those things that in order to really have lasting gains it actually needs to do very consistently (huberman also did a podcast on this very topic), it does sort of have temporary improvement in individual sessions but it quickly goes away if not done for a certain length of time & not done basically daily (use it or lose it)...doing full range of motion resistance/strength training is actually more effective, as it actually builds strength by providing load throughout that range of motion vs stretching that basically just allows for a temporary improvement in range of motion