Monthly water changes isn't going to be a problem for you. I am concerned more so how exactly you are dosing your BS and Calcium. Also, what exact products are you using? I like the B-ionic 2 part additives. I'm currently running a calcium reactor, so I don't dose except to make corrections from time to time. With your PH being what it is you might consider dosing kalkwasser. There are many methods of doing this. You could set it up for a once a week thing with a 5 gallon bucket and some polytube. Provided you have a place to house the bucket. SUMP????
Mrs. Wages pickling lime is a very inexpensive solution for kalk dosing. Also, there is a two part that you can buy on line that is getting good reports, I forget the name exactly. But it is sold in buckets and 1 gallon jugs. I want to say it "Total two part" I found that googling around, but I cant be sure that's the correct stuff. It would be a fine alternative though.
Now the thing is, the shrooms and other softies shouldn't give a hoot about calcium carbonate. They have no stony structure. so something else is amuck here. What is your water source? What lighting do you have? Are the bulbs new or less than a year old? Evaporation is good. It is an indication of good oxygenation(or at least gas exchange). The room that the SWA is in, is it a real tight room? Can you run your skimmers air line to an outside source? If so, increase the line size some to accommodate the extra resistance it will encounter from doing so. Also, make sure its in an area that will not become contaminated source. Like around vehicle exhaust, or possible animal waste(saw your avatar was horses) etc. etc.Do you run carbon? If not, I would recommend it at least periodically.
You appear to be a fine Aquariuest, I have been in the same similar spot. You'll get there, we just gotta figure out whats going on. Also, any changes you make, make them very gradually and ONE THING AT ATIME., Otherwise you might not ever find out exactly what is going on. Additionally, dosing alkalinity and calcium should be done DAILY(it only takes a second) you don't have to test daily, just find your daily usage and dose that, testing monthly. When adding be sure to mix with RO water and add VERY SLOWLY, with at least one hour between alkalinity and calcium. You should have a grasp on your usage by taking what you dose to get to where you want to be and dividing that by the days between your last dose. This will get you started finding your consumption rate.
However...Mr's wages and a bucket would be much easier.