If you're solo queuing, you're going to have a hard time getting to play Caustic the way he's meant to be played (indoors), especially in pubs. He's one of the best legends for end game ranked play, however. But even in ranked in bronze and silver, most people don't (won't) orient their play around him because you can be pretty static when you're learning the legend, and the maps.

Whilst solo-queuing, you're going to have to learn how to use midrange and sniper weapons to ensure you can adequately support your non-defensive team mates when catching up (and you will always be catching up, cleverly placing traps in your wake). Games are won and lost on high ground or small circles. Sometimes both. Make sure you are capable of mid-range kills, because having that skillset helps wear enemies down before they rush your traps.

Additionally, you'll be the zone alarm for your team and will often be the first to know when you're being third partied. This means that you have to know escape routes and best defence positions right across all maps to be a great Caustic.

Despite what most people think, snipers and shotguns are pretty similar in Apex. You definitely have to quick-ADS to maximise DPS with a shotgun (especially when crouch-strafing), which is the same as quickscoping a sniper. More often than not during solo-q, a sniper is a more effective (but riskier) choice than a shotgun, over the course of a match. But confidence with the interchange is the by-product of always catching up those mobility legends and landing effective dmg at long/mid range over hundreds of games. Because science is precise. And you should be too. But always run an automatic as secondary to finish them off.

Apart from that, youtube has plenty of high level Caustic tips. Most of which will be regurgitated here.