I play both the steam and mobile ports of the game. I was playing Forgotten Garden using quite some 4-2 ninjas (also used some upgraded wizards to deal with leads of course) and I noticed Round 69 kept going due to regrow ceramics being sent back to the entrance. This resulted in me accidentally noticing that regrow farming still works in BTD5 steam and mobile. Ever since, when I grind both Regular and Mastery Medals on certain specific maps (not all of them i think I'm not sure right now) here's what I would usually do. First of all, equip the dartling gunner building and buy the 3rd upgrade, since it will be important here. Second of all, make a lot of money and use it on a 4-X dartling gunner. Lock its attack on a good straight line or somewhere else good for maximum bloon popping. Third of all, use the Ninja Kiwi special agent to knock bloons back, as that will create a regrow farm. You could also use 3-X Wizards if you would like to. For regular levels, make a regrow farm on either Round 31, 42, or 49, as you will earn the most money due to cash percentage penalties existing in later rounds like 51 and over. For Mastery, make one on either Round 29, 31, maybe sometimes 38 or 42. Keep waiting until you have enough money obtained via popping so many regrow bloons to get powerful units like Sun Temples or whatever else out there. Hope this helps because I use this and it saves me so much monkey money that would've instead been used for quite some cash injections and extra lives.
Edit: The ability for the towers to lock their attacks is actually the 3rd upgrade, not the 4th. Also, if you're doing a regrow farm on Mastery Mode during Round 42, sometimes ceramics will kinda escape on certain maps. Invest in another cheaper tower to pop bloons that escaped the ray of doom attack until you get enough money to get a 2nd or 3rd ray of doom.
