Ms-SQL optimising

Hi I have an MS SQL database, not massively big, but with some core functions that need tuning/optimising to increase performance. it was originally designed on SQL2005, migrated to 2014 last year and now to 2019. However we have had to turn on cardinality optimisation as the performance was awful with it off.

I'd like guidance / work on indexing, partitioning, and maybe some stored procedure / query re-engineering to make use of better features in 2019