The LTR must be one of the simplest thai visas that one can get, though I only have experience of a tourist visa (in Vientiane over a decade ago) and non-o retirement, both of which I managed to do myself without any major trouble.
With LTR the big thing is you don't need to go visit immigration office multiple times telling you that something is not as they wish even if according to official requirements). Instead you just upload the pdfs of required documents to their website and if they ask foe something more, upload those. No hassles with immigration officers with varying requests and going back and forth to immigration and waiting on queues.
If you are able to get the required documents (that agent can't do for you anyway), you must be able to upload them to their website too.
I can't see any added value from any middleman, just additional hassle.
As a bonus you get rid of the 90 day reporting and from keeping your cash locked into bank account - not to speak about the tax worries