As mentioned, don't let the Masters 45+ fool you, those guys are strong af. So unless you're really strong and just don't race a lot, I'd start with the Cat5 race and see how it goes.

I did my first (and only lol) road race last year in Cat 4/5, and did well. Same race is coming up again next month, I've improved enough (I'm not licensed, but would prob be a strong Cat3, maybe hit Cat2 with more focus/racing), so I'm going to do Masters this time (I'm 46), and it will def be a tough race.