The sediment and carbon block filters have very little to absolutely nothing at all to do with TDS, you don't change them based on RO or RO/DI TDS readings. TDS is dissolved solids in the 0.0001 micron range. Sediment filters remove or trap suspended solids or TSS in the 5, 1 or whatever micron range, big floating stuff not seen by a TDS meter.
You change the sediment and carbon block every 6 months as a general rule. This is mostly based on an easy to remember date or number and not scientific. If you want to get more precise you need an inline pressure gauge, or better yet two, one before the sediment filter and the other after the carbon block or like many system in the membrane housing. You monitor headloss or pressure drop and when it drops say 5 psi you change the sediment and carbon block filters. The other thing you need is a low range chlorine test kit to monitor for chlorine breakthru from a plugged or exhausted carbon block. Good 0.5 or 0.6 micron carbons are supposed to last up to 20,000 gallons of normally chlorinated water but if the sediment filter allows silt, particulates and colloidal materials through it plugs the billions of tiny pores in the carbon structure rendering it useless for chlorine adsorption.
As you can see its easier to stick with the 6 month rule for most people who do not want to spend the money or extra time monitoring filter conditions. You also should didinfect the system at the filter changes to keep it clean.
I would suggest a better sediment filter so it does not foul the carbon. 5 microns is very coarse and it is probably a nominal rated 5 microns not even an absolute rating so even worse. I would suggest at least a 1.0 micron asbolute rated sediment filter if not a 0.5 or 0.2 micron version which is what I use myself. The reason I say this is you can actually see 40 microns with the unaided human eye so 5 microns is pretty darn big and passes a lot of small stuff through to plug the carbon. With a better sediment filter the carbon block will last many times longer between changes and save you money even though the sediment filter itself is a little more expensive than the old 5 micron version.
You change the RO membrane when it is no longer cost effective to keep replacing DI cartridges or resin, I stay close rto 95-96% as for every 2% drop you cut your DI life in half. The difference between 98 or in my case 99.4% RO rejection rate and 95% is significant when it comes to DI replacements and a new membrane pays for itself quickly. You will also discover better sediment and carbon block filters will do a better job of protecting the membrane from suspended solids, not TDS or dissolved solids, and the membrane will last longer since it is cleaner.
You replace the DI resin when you first start seeing signs of anything other than 0 TDS on a regular basis. The reason being some substances such as phosphates, silicates and nitrates are weakly ioniozed, not removed well by RO alone and not always removed well by DI resin once it starts becoming exhausted. The resin will start releasing the weakly ionized substances at much larger levels than it was in the tap water since it has been concentrating it in the resin and a TDS meter may not detect all of it since it has weak electrical properties and thats how TDS measures, by conductivity.
