This is Kiran, I have 6+ years of experience in Sencha ExtJS and Java.
Can help you to estimate and i can also take up the work for migration.
I have recently migrated a sencha touch application to ExtJS 6Lisää
Based on your requirements, i have strong feeling that i could be the best fit for your "ExtJS 5 Grid Multi-Level groupping" job for the following facts:
-] I have proven experience & deep understandLisää
I work with ExtJS for more then 3 years , from version 3.4 to 6. I have experience with code review.
I don't have experience with Java, but i think the review will be only on ExtJs code. Depends on the size of the prLisää