Extreme low rh

You need a humidifier mate ! If you can’t have one, you can’t using a sprayer couple of time daytime, be carful to not spraying your leaves if you do it

I wouldn't worry. Low humidity is good for flower. Now if you dry in those same conditions, you might dry a little more quickly. Watch for overdrying. I still wouldn't pump in extra moisture though. It's better to run dry than humid.

i d be happy if my rh was like yours in flower, mine is often over 50% and it's when i start having some mold with some strains and phenotypes within those strains.

30% in late flower is good, not a problem.

Let the water run off into the cups, this will slightly increase the humidity under the foliage.

Why don't you use a humidifier?

You problably know you need a humidifier, which i go into below.. but if you "can't" do anythign for any reason rational or irrational, you can mitigate this slightly by reducing concentration of feed to match increased uptake of water in plants. A high VPD environment resutls from low RH. This results in more transpiration occuring per increment of time and that increases the amount of water the plant drinks per day. if it is drining 20% more than before, you shuld drop concentration of fertilizer to math. that way you are providing the same mass of bulding blocks per day as before --- this assumes growth rate was good and everyhting else was/is fine. now trackign how much water the plant used will help inform, but also consider how much large plants have grown.. this makes it very unclear, lol.. just a guess. But, keep eye on plants if they start looking too lush or start clawing, you'll see that reducing concentration of fertilizers is needed when VPD spikes. you'll still get better growth rates with more approriate VPDs but you'll avoid the toxicities. which is obviously a net positive effect. ------ humidifier... evaporative cooler.. something.. the "cold" humidifiers are better than the ones using heat or atomize as far as avoiding a mineral dusting of surface area. some "demineralizing" additives can help with that too. the atomizing ones don't work that well, if i recall? those are the ones that leave a filthy residue on everything too. cold just means it's a fan blowing over a wick that increases surface area while soaking up water from a reservoir. A fan over a bowl of water can do it too, but soemthign that increases the surface area will drastically improve the effect. These type of units, if purchased, do not require many watts for operation.. just a strong fan. Need to replace "wicks" occasionally. you could diy something... hang some towelst that dip into some water and blow air between them.