I'm not going to tell you that I think your canister filter is the root of your problems, but I will say that I believe it to be at least a contributing factor.
I think daily cleaning of a canister filter is a bit much to expect from even the most anal retentive reefer, but monthly is WAY too long, and if you aren't even keeping that schedule.......
A sump is never really
necessary but it's a really nice thing to have. The added water volume alone makes it good to have, and the ability to hide hardware that you don't want to see in your tank is a bonus.
It's good that you're using RO/DI, are you monitoring the output water for TDS?
The fact that you are growing coralline is a good sign, how 'bout that nuisance algae?
Low PH is also not likely to be the entire problem here, but it needs to be fixed in any case. Are you sure that your PH is low? You said that low PH is a new thing, and it's always been high. Could be time for a new test kit, or a new probe. Lots of people chase their tail trying to fix a PH problem that doesn't exist. Get a second opinion from someone else's test kit before you do anything.
I am a believer is skimming. Keep your eyes open for a good used skimmer. I find that anything hand made by
Geo,
Barr Aquatics or
Lifereef beats the hell out of anything mass produced to sell at LFSs.