Welcome to BF and welcome back to racing. First, I assume that you are still a cat 3 or USAC still recognizes you as a cat 3. Yes, there are Masters 55+ races. Generally, they are open to all categories or the promoter will specify. For example, there may be a 55+ open, 55+ 1/2/3 and a 55+ 4/5 or even a 55+ 1/2/3/4.
If you are still a cat 3 then you will have to race in the open or 55+ 1/2/3. Generally, there is no 55+ Cat 3.
The 55+ 1/2/3 will be a very hard race. There is not much difference between Elite 1/2 and the 55+ 1/2/3. It depends on the region and who shows up. And you can always race with the younger guys which at times is a better deal. Generally, the masters are a little smoother / refined but then again it depends on who shows up.
If you want more racing per week, you can race with the Cat 3 men, any of the younger masters categories such as the 45+ 1/2/3 and the 55+ 1/2/3, So depending on the timing of the races you could do three in a day or any combination thereof over two days.
And you can race masters age group category races at state, district and world levels as well as events such as the senior games.
In general, racing opportunities are being reduced due to a lot of reasons. But new racing opportunities are gaining popularity such as cross and gravel. IMO, this is cyclical and today, there is no catalyst to drive more USAC style racing and inspire promoters and new racers.
Also, check out the 33 which is where most of the racers are posting on BF independent of age.