I think clean/lube of your shifters should be top of the list. I use WD-40 to blast out the gunk, others use brake cleaner then a lube. Removing the plastic cover (or covers) on the bottom of the shifter is critical, this exposes the delicate pawl and ratchet guts so you can shoot the cleaner right in there while operating the levers. IME, this means taking the shifters off the bars, and ack, the bar tape too. The hoods come off too, they don't love WD-40 or brake cleaner.
Shimano Dealer Manual - cover removal on P23
https://bike.shimano.com/en-EU/produ...0/ST-6800.html,