Jazz is one of the most sophisticated genres to improvise over and it can be seen as a style to add in the most theory as possible that not only sounds good, impressive or inspiring for other conscious musicians listening, but to also to sound nice to the average music listener who has no experience in music.
The scales that you are already playing are nice. But as you said, it can sound a bit monotone due to most of them having similar notes. As I said in the comments, in jazz players tend to not rely on modes for improvisation.
So, what is another improvisation method?
To use your arpeggios, which, as you already know, is just individualising the notes of a chord. e.g. in your case of playing the chords Gmaj7 - Cmaj7, you can play your Gmaj7 or Cmaj7 arpeggio (pretty obvious).
Arpeggios are good because you're not just improvising over one or two scales throughout a song, instead you have to change your improvising structure each chord change and take into account majors, minors, suspendeds, additives, augments, alterations and substitution! :D The possibilities are endlesssssssssssssssss!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
A good friend of mine Jens Larson makes an awesome playlist showing you how to use arpeggios to improvise. But I recommend that you learn some of the theory from the links that I am yet to provide before you go to his playlist. Just so it's not too confusing :)
!From here on down think of all chord structures as arpeggios eg. Gmaj7 = Gmaj7 arpeggio!
After knowing some basic arpeggios you can then add in tritone substitution. Here is the most common example where you completely substitute out a chord (or arpeggio)
for another (we will use the simple standard ii - V - I, make sure you think about using these as arpeggios and not chords)
Dmin9 - G7 - Cmaj7
Standard with substitution:
Dmin9 - Db9 - Cmaj7
Where the G is completely substituted out for the Db.
You can also add in Alterations where you add in other notes to create more tension in improvising which can sound nice if your bass player and other instruments are sticking on the root chords(It's not going to sound nice if the whole band tritone substitutes, you need a root notes in order to substitute off of). I'll use the same standard as before for example purposes.
Standard with Alterations:
Dmin9 - G7b5#9 - Cmaj6/9
There are a literal tonne of different alterations that you can add in that depend on the song. The most common (and easiest to first learn) is
b5, #5, b9 or #9 alteration. Justin Guitar does a good video explaining a lot about alterations here. Just make sure you arpeggiate the chord structures in which you will be learning in order to improvise over. After watching that video that explains basic alterations, here is a link that explains all about it in depth.
EDIT: Depending on what kind of style you want out of your improvisation, you can learn how to pop and bop which is to change up the pronunciation of each note and to add pauses, cuts and holds to add unusual timing that somehow sounds appealing. At the start of this jamming track by QuistJam he does a lot of popping and bopping. It sounds awesome!