You are correct. With the lump sum option you do not have to transfer any specific amount on any particular schedule, just whenever you feel you need to bring the balance up. This assumes you use said account for expenses such as rent, which I do. So it does need to be replenished occasionally. Hope this makes sense.
The monthly income route seems way too complicated and the rules very strict, thus I'm glad they offer the 800k lump sum option. But to avoid dipping below that amount around visa renewal time, I use 900k with no worries of meeting day-to-day expenses. Just my method for ease and peace of mind.
I don't have advice for an agent there because I did mine in Bangkok. But I applaud your choice of utilizing an agent. It made things so much easier and faster. All done in under a day with no stress. It costs more but money can be made and time cannot.