It's best to create a lilac tree without grafting by starting with a whip and maintaining a single stem until the desired trunk height is achieved.  Lateral branches are then allowed to grow but are pruned strategically to achieve symmetry or other desired form.

The overgrown lateral branches above the relatively short trunk shown on the specimen in the photo must be reduced and controlled to achieve your objective. Doing that will be a multi-year process best done under an expert's guidance.

Under the circumstances, we recommend addressing your question to experts at the International Lilac Society. 

https://www.internationallilacsociety.org/
https://ask2.extension.org/scp/tickets.php?id=792078
  
Information at the following websites may also be of interest:

https://gardenlux-en.designluxpro.com/sad-i-ogorod/dekorativnye-kustarniki/shtambovaya-siren-foto-sorta-posadka-i-uhod-primenenie-v-landshaftnom-dizayne.html

https://www.gardeningknowhow.com/ornamental/shrubs/shgen/pruning-a-shrub-into-a-tree.htm