Getting the plastic inner plate (the "Set plate") thing back into the right hand sideplate with everything in the state it's supposed to be in, ugh.

Also the bearing the spool shaft runs on can get very very stuck in the sideplate, at both ends. If you decide to do the spool bearings, be super super careful removing the spool pin on the right hand side, it's very easy to slightly bend the spool shaft and then you're up for a new spool.

Schematic is here:

https://www.shimanofishing.co.nz/images/schematic/TRBCAl4-CALCUTTA400B1.pdf" rel="nofollow - https://www.shimanofishing.co.nz/images/schematic/TRBCAl4-CALCUTTA400B1.pdf

A common mod is to replace the driveshaft bushing with a ball bearing.

Also worth checking if you have the updated clutch bar assembly (see bottom left corner of schematic) - although good luck getting one out of Shimano at the moment.

Watch out for corrosion under the handle arm, and behind the handle arm nut retainer.

Be careful not to get grease on the anti-reverse bearing or it will slip. Lightly oil it at the most.

If parts were actually available... I'd also consider replacing the handle assembly with one off a Calcutta 400BSV. Big upgrade over the paddles, especially if yours have gone "gummy".