Men take only their needs into consideration - never their abilities.
Napoleon Bonaparte
Look there is no square-space for spectrum game development. If there were such a thing people would be churning out fappy bird clones on a daily basis.
You got to put some effort into this, invest some time to get results. Even the game creator tool you still have to put the time in to learn how to use it. It's all downhill from here on in terms of effort/time. Basic - little bit more control but a bigger learning curve. Then assembly - steep learning curve,
You mention plot and circle commands.. If you are going to do stuff in basic then think about putting the values in an array and using a for loop to build the screen. Will be slow though..
Assembly you create your own data structure through code, define the memory location you want to use - splurge your data all over it and then run the subroutine stepping through the data stored in these memory locations. You can use the routines in Rom to do this, but if you are feeling brave you can write your own plot routines.
At this point though you are saying 'I am writing my own graphics engine...'
Cross platform development was pretty common. Especially if there was money on the line it made sense to develop on the system that gave you the greatest advantage that you had access to. If you are planning to do everything on the spectrum just from basic that is going to suck.
Software on the spectrum .. yea I mean its possible but you are making a rod for your own back.
Stuff like Grafx2,spectrum paintbrush, SevenUP are way ahead of anything you can get to develop on the spectrum. Time/Quality/Features you win big creating this stuff on a modern system.
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Listen carefully to Smith is saying, he was worried about if people paid for the game, he had a tight development cycle and used a development process/tools that enabled him to create games in the most efficient way possible. If one of the top developers of that era took this approach and was so focused on the $$$ more than some 'author/artist' expression idea. Imagine what it would have been like for the B/C/D team guys who just wanted to churn out as many titles as possible.