I Tried Sidemount for the First Time

[DevonDiver]

The underarm bungees were a little tight...

Bungees need to be quite taut in order to provide the correct positioning of the valve-end of the cylinders and enable proper cylinder trim.

What you can do is investigate different bungee materials and designs - some with more/less elastic properties, old-school configuration, continuous configuration etc. You may find you have a preference that works best for you.

...and I had trouble with the remove and reconnect drills.

This is, in my experience, what most novice sidemount divers have the most problems with. It's largely an issue of muscle-memory and hand coordination. It can cause more than it's fair share of huffing and puffing initially - but soon becomes an easy routine skill.

That said, choice of bungee material and design can make things easier or harder. Likewise, there's a few 'tricks of the trade' that help make connect/reconnect bungees around the cylinders much easier as a skill. A well-versed, expert, sidemount instructor should quickly put you to task with these...

When one adds a 40CF EAN50 deco bottle and perhaps an O2 bottle, how difficult is it to get to the leg pockets to deploy a DSMB or retrieve the spare mask?

As others have mentioned, one popular solution is to carry items in a detachable pouch (see here for example) - secured to rear D-rings with double-end snaps. Thigh pockets are problematic with sidemount and stages/deco.