This is not explicit really. It doesn’t say explicitly that he blew up the planet in his first form. All he said was that he attacked planet Vegeta and killed the king which given the context could more logically be interpreted that he was talking about attacking and killing the people of planet Vegeta. Like Goku Black attacking the Earth - the humans on Earth. Not literally throwing an attack on the planet itself to destroy it. It is entirely possible that after Freeza killed king Vegeta and several Saiyans with his first form he transformed to his final form (while nobody was around to see him) and blew up the planet.
I take it as being more simple than that, that the entirety of his time attacking the planet, which included his destroying the planet, was all done in his base form. You say that the panel of the manga showing him killing Bardock with the blast doesn't count, but given that Toriyama based those panels on the anime special, and that Bardock died in the special with the same attack that destroyed planet Vegeta, there's no logical reason to assume that Toriyama aimed for something different in his depiction of the events for that particular scene.
And using your logic in this case there are things shown later that discredit Freeza being able to blow up planet Vegeta in his first form. At least not instantaneously like Roshi/Piccolo did with the moon. We saw 50% Final form Freeza thowing a big blast to destroy planet Namek but the destruction would happen after 5mins. And though it is implied that he held back, it is also implied that destroying Namek instantaneously would be pretty much his top feat. And later King Cold remarked that as Earth is such a small planet they can destroy it in one shot. And when he said so Freeza was in his Mecha form which was stated to be stronger than his final form. Also apparently for other bigger planets than Earth they would need to do more than just firing a single shot. Planet Vegeta has 10x bigger gravity than Earth and as a result it needs 10-100x times more energy to be destroyed than Earth.
If we want to get into mass and density to determine planetary destruction, then even then it'd only take someone in the hundreds of thousands to destroy planet Vegeta. The moon has a mass of 1.2% that of Earth, and Roshi destroyed it with a battle power of 137 (perhaps greater, but this is a baseline battle power we know for him). Earth then would require a battle power of ~11,000 to destroy it, fitting with Vegeta being able to destroy it with a battle power of 18,000. 10x that of the power needed to destroy the Earth and you get ~110,000, which means that Freeza, in his first form, could casually destroy it with his battle power of 530,000.
Density wise it gets even smaller. The moon's density is 60% that of Earth's, so if it takes 137 to destroy the moon easily, then it'd only take ~228 to destroy the Earth, and thus only 2,228 to destroy planet Vegeta.
Additionally, I fail to see how it's implied that destroying Namek instantly would have been his best feat. All that's indicated is that he held back a good deal of his remaining power (he was already weakened from the Genki Dama and his damage from Ssj Goku) and still had enough power there to damage the planet to the point of it blowing up in 5 minutes. Nothing about him destroying it instantly being the best he could do.
If they wanted to explain about Shismai getting stronger with Frieza they easily could have. Frieza wasn't even surprised when Shismai got one shotted. Not only did it explain that Tagoma trained with Frieza, it also mentioned multiple times how he got stronger. He then stomped the z fighters. All Shisami did was hold Piccolo for 10 seconds and then get one shotted by Gohan.
Again, you're overanalyzing things. Which is the simpler to accept, that Shisami trained during the same length of time that Freeza did and grew strong enough to give Piccolo a hard time? Or that Toriyama somehow believed that Piccolo was so low strength wise that an individual around the 20,000 range could give Piccolo trouble and that Gohan would need Ssj to beat? Even without knowing the whole Tagoma/Shisami situation behind the scenes, it's far easier to just extrapolate that as the likely scenario, that Shisami trained.
As for the whole Z Senshi thing, as said, the Z Senshi were clearly suppressing themselves to buy time for Goku and Vegeta to show. Any "difficulty" they had is attributed to them holding back, not legitimate weakness.
Gohan trained for years and didn't even surpass Frieza until a year of intense training in the room of spirit and time. Goten and Trunks are a couple years younger than Gohan was then without the long intense training. And like I showed ssj Gohan was cared of a rock base Goten threw. So being afraid of their attack doesn't make you weaker.
He was easily well above where you are putting the boys base battle power wise before he entered the Room of Spirit and Time. The only reason he didn't surpass Freeza at the time is because he hadn't transformed.
And, as said, the manga and Daizenshuu readily point to Goten and Trunks being prodigies even in comparison to Gohan, so what might have been difficult for him came easier to them, including the quickness that they grew strengthwise.
