It is all completely a personal preference type deal. There really aren’t any wrong choices - what ever you the shooter are comfortable and “confident” with.
The top shooters that consistently win do not vary much at all - 1-1/8oz, 2-3/4 dram shells for singles and both shots of doubles, and handicap shell of choice (3 dram equivalent) for handicap.
My Dad used to prescribe to the mindset of shooting 1-1/8oz 8’s on singles, and the second shot of doubles, and then 1-1/8oz 7-1/2’s 3dram equivalent (reloads with less felt recoil, but just as quick) for handicap.
We both generally are happy shooting 1oz 8’s for both shot of doubles these days, with the mindset that you know where both targets are going to be, especially the first target, and generally speaking, because of this, doubles targets are “generally” shot at a shorter or closer distance. This may not be the case for a newer shooter, not having the target experience.
I have gone to shooting my 1oz reloads for both shots on doubles and all singles, and the aforementioned handicap reload, for handicap. I believe that there are more pellets in 1oz of 8 shot than there are in 1-1/8oz of 7-1/2 shot....reloading 1oz is easily more cost effective.
One other aspect is your individual taste in reloading, for tinkering. Nothing and nobody says you can’t have as many different reload recipes as you like. Me, I prefer to minimize, which has me making reloader changes and adjustments less frequently, and that all works for me....
When reloading costs are close to buying factory, I will buy as mamy 1-1/8 oz 8’s that I can get my hands on, and shoot them on singles, both shots on doubles, and handicap back to the 24 yd line.
I have known shooters to get all the inexpensive factory shells they can, and shooting 3 dram 1-1/8oz shells for everything. The only caveat with doing this is staying in the gun, given the recoil, so unless you have an excellent recoil reduction device, this approach may not yield your best scores.....
That all said, most of this topic truly comes down to personal preference, as well as reloading component costs, enjoyment in tinkering with reloading, and confidence in the shell when shooting. As many have pointed out, much of trapshooting is mental.....