The circuit is an interesting game mode. The RNG or randomness allows players to explore new, retired, or unacquired weapons and frames however, if the player decides to play steel path, they will need to determine whether to play with the weapon unoptimized (unleveled, missing catalyst, forma etc.) putting them at a huge disadvantage to start. Or instead, the player can leave Duvuri and try to level and optimize the gear by other means first. Then revisit the circuit knowing they will have a better chance at creating a more optimized build when those weapons come back into rotation.
How I wish the game mode worked is that the player had the option to select the frames and weapons they want to use from the start. The game mode is not endless instead there are different variations. For example, the circuit game mode variations offer a set number of stages such as, 3, 6, 10+ that the player could opt into from the start. The player will be allowed to choose their frame and weapon for the first stage. Once stage 1 is completed the frame and weapons used during that stage are locked out from being selected in future stages for that run. To start stage 2 the player will again select their next option from a pool of weapons and frames they have preselected. The preselection phase would start once the game mode is selected. The player will enter a solo lobby to start preparing what weapons and frames they want to add to their pool. Once determined they will enter a portal to start the mission or if playing in public, they would instead start the match making process to start the mission. The initial setup ensures other players cannot start the game mode prematurely while others are preparing their loadouts. The option to select the number of stages will allow newer players to get in on the action after they have acquired a few weapons and frames due to the limitations of their arsenal. For example, stage 3 circuit being the lowest tier available would require that the player have 3 unique weapons and frames which I believe adds more relevance and promotes the idea for players to collect more frames and weapons. In addition, the player will have some experience playing with the frame and weapons prior to having access to the game mode. More seasoned players can take it further as they have acquired more weapons and frames by playing higher stage circuit modes allowing them to put their arsenal to the test while promoting the idea of collecting weapons and frames. This also promotes player choice and does not undermine progression. The last stage of every circuit run would have a boss fight. Longer runs if steel path can have additional twist and turns added to increase level of difficulty to keep things interesting.