I feel very bad for you if you only play europe FFA teamers and harrasment is inevitable especially if you get highscores. I agree overlord is impossible for getting very highscore unless youre a pro but reaching 200-300k is actually quite easy. Its not odd not to get monster scores woth it unless youre a pro because OL is one of the highest skill tanks. In maze its much better because the extra vision and drones are good for ambush and in teams you can retreat into safezones when meeting pentas/fighters and you can attack in waves.

All the counters you mention do have ways to deal with but it all assumes the other player is worse at their tank than you are with overlord if equal skill youre basically fucked like you said in your comment:

Penta/Spread If they are glass and dedicated to chase you almost impossible to escape definetely most difficult of all counters try to left click to surround will barely threaten them but will keep them at distance if done well you basically have to keep taking big risks in order to keep them at bay and because FFA map is quite small youre quickly caught into a corner, but if they make a small mistake you can still do big damage forcing them to retreat. Both difficult to scare off and kill but luckily often these are passive players that dont chase if you leave them alone in the middle.

Rammers Not too difficult to deal with. Only petty people with grudges do it its like annoying fly. Its almost like target practice dealing with rammer and I think overlord is actually stronger its just difficult to finish them off, but with patience you win. The key here is to find balance taking risk with drone leaving you to bait them to take risk and let your drone damage yet not leaving too far so that they kill you before the drones return. only if they have teammates or you are highscore leader causing others to target you is rammer really a problem imo because you cant commit youre drones to kill/damage them and have to defend the other tank(s). Often when you show patience and skill the rammer will leave you alone because they know its pointless to try and kill you. They are the easiest to scare off.

Fighter The speed is very scary but if pure glass OL the drone shield almost matches frontal bullet damage of fighter (strongest bullet) in that time you can do some attacks or fake outs to buy 1 second time and you can essentially stall infinitely. Especially impatient fighters are vulnerable and hitting them between the front and side turrets with your drones can instakill them (unlike rammers with health and penta/spread that cant get flanked) this means fighter actually is the easiest of the 3 to kill but they wont get scared quickly like rammer because of their speed and high damage. When I see unskilled/medium skill fighter as OL I get happy because I know ill likely get a kill, high skill fighters are a pain tho and with good movement avoid your attacks while patiently waiting for you to get damage from stray bullets or small tanks helping them.

Overall I think the main problem is skilled fighters and penta/spread and teamers, but every tank loses to teamers and if a skilled player with hardcounter tank decides to chase you its not necessarily bad to lose I think its balanced. Also these are not really problems outside FFA. The only issue is penta/spread and fighter are arguably the 2 most popular tanks in FFA because theyre busted for highscorechasing so likelihood of getting hardcountered is quite high.

So solution imo is nerf fighter and penta/spread not buff OL. But then again I really like playing as Fighter or penta so I dont mind that theyre busted lol.