While learning songs shouldn't be your sole priority, I think that learning repertoire should still be a fairly substantial part of your musical development. Done right, it should also guide you towards what you need to practice. If you try to play a song on guitar (for example), and find that you can't make the chord changes fast enough, then you can turn those difficult changes into an exercise for yourself.
This is also good because it gives you a tangible sense of progress. It is often difficult to find motivation if all you're doing is playing abstract drills or exercises, but when you're instantly able to apply what you're learning towards a new song, then you'll have a much stronger sense of becoming better at your instrument. So instead of trying to figure out which techniques to practice, perhaps start out by figuring out what you want to play, and then think about which techniques will take you there.
Since you mention wanting to improvise, I think one important thing (which is often neglected by beginners) is to work on developing your ears. To be good at improvisation you need a strong connection between your ears and your fingers (so to speak). The ideal (which will likely take some practice) should be that you can imagine a phrase or melody in your head, and be able to play it on the instrument almost instantly.
One exercise you could start with to develop your ears is recognizing intervals. There are a lot of websites/apps/programs which will play you two notes and ask you to identify the interval between them (for example: here). Since you have been studying theory I assume you know about intervals already, but you should be able to identify them by sound as well. If you're new to this I would start out with only two or three intervals, and add more as you get comfortable (maybe start with perfect fourths and fifths, then add major and minor thirds etc.).
Another thing you should try working on is to transcribe music that you like. This is tricky if you haven't done it before but it's very worth it once you get going. Start out by finding some simple song that you like and try to work out how it's played just by listening (if you have a way of writing it down that would be good too). Figuring out chords is more difficult when starting out, but you should be able to make some progress with simple melodies.
Once you've worked out how something is played, you can use your study of theory to analyze it and figure out how it works. You may ask yourself things such as: Does the melody fit into a particular scale? Which notes are emphasized and which are avoided? How does the melody relate to the underlying chords? Does it mostly play chord-tones, or are there some "outside" notes as well? Are there any recurring melodic phrases or rhythmic patterns? There are endless things you can consider, but try to focus in on the things which make the music sound good to you. This doesn't only apply to the "theory" part the music, but will also be a good way to pick up on things like phrasing and dynamics, which are equally important in making something sound good.
When you can put a name to the things you like, you'll be able to incorporate it into your own playing. For example: say you analyze a guitar solo (or whatever) that you really like and you notice that the guitarist has a tendency to target a certain note in the underlying chord (the 3rd or 6th or 9th or whatever it is), then you can easily turn that into a practice exercise for yourself. Simply figure out where those notes are found on the instrument, and practice improvising while targeting those notes. That way you'll both expand your own vocabulary on the instrument, and develop a stronger sense of hearing.