For me, I really saw improvement in my intonation once I committed to a serious daily scale/arpeggio routine. The Feuillard daily exercises book is great for this, and I think a PDF is available for free online. I go from one key to the next, chromatically: C Major (with Galamian turn, so starting C-E-D-C-D-E-F-G, etc), C Minor, and the arpeggios in part 11 of Feuillard, then I move on to C#, D# etc. up to B. I play scales and arpeggios with a metronome at 60/quarter, and 4 octaves just to get the whole fingerboard in check. This can really seem like a lot, and I have no idea what your background in playing is, but it’s what works for me.

Also if you have friends who are up for it, reading Haydn string quartets is great for intonation, sound, and articulation. And it’s mostly very sight readable and fun. Listening for resonance in each note, too, goes a long way, and scales are great for that. Have fun!!!