Don't think Trump was officially the nominee until the Republican convention, and the delegates officially name him.
But, he has had a detail since being president. His detail asked for more support, but was denied....hence the USSS firing of director and others.
Since the USSS is under the DHS umbrella, they get supplemented with HSI SA's, and other gun carrying LEO's that fall under DHS. But, the election period is a huge manpower suck and is one of the reasons USSS SA's have retired, or moved to other agencies....
As far as the DSS, they are far too small and the bulk of their agents are OCONUS, as they are a foreign service. The do have a PSD team, and tactical PSD team that travels with the head of the State department. They have lead on the UN General Assembly that happens every year, and they pull from a lot of agencies for assistance, a HSI friend of mine has had to go for the past five years.....and do Texas border TDY's and either the DNC or RNC every election year, he feels like he is pulled in too many directions.
Maybe having the USSS be purely a protective service might be a good idea....
Unfortunately, violence toward elected officials seems to be the way crazies go now.....just think how hard it is to have a protective bubble in open areas you don't fully control, and your principle has to get out and meet and greet.....I would hate that job.