Again I'm all for investigating whether war crimes have been committed by Israel. It's going to be a nuanced argument in any case to prove so that will probably involve how many civilian casualties are acceptable to achieve legitimate military aims.
The contrast must be pointed out by all who want nations and non state actors to be accountable for their actions.
Instead, the case is mostly about intent, and that can be gaged from public declarations and actions outside of mere combat. The case against Hamas is clear, they attacked in secret, with quite likely no military targets at all, and with a clear history of anti-civilian sentiments and declarations.
The case against Israel is also relatively simple from this point of view: numerous Israeli leaders, from the president to ministers to members of the Knesset have given public declarations about the collective guilt of Gaza's civilian population, and their actions in preventing aid from entering Gaza, attacking refugees, attacking journalists and international aid workers have been thoroughly documented.
If you think the prima facie case against Bibi and Gallant is convincing, the Israeli AG is quite plausibly doing so little that subsidiarity is no longer engaged. If you think it is unconvincing, as you say, the problem is not some inappropriately symmetric ignoring of subsidiarity but that the charges themselves are unconvincing.
A final point is that the Rome Statute does not prohibit merely ‘orders of directly targeting civilians’, and so other potential crimes must be considered. These include ‘cruel treatment as a war crime contrary to article 8(2)(c)(i); [e]xtermination and/or murder contrary to articles 7(1)(b) and 7(1)(a), including in the context of deaths caused by starvation, as a crime against humanity; [and o]ther inhumane acts as crimes against humanity contrary to article 7(1)(k)’. Of course, you may think that Khan has jumped the gun on each of these in that each of these charges is also implausible, but that is a stronger position than doubting that there were orders to directly target civilians.
(edit: I should add that Khan [I imagine] and I would say that while subsidiarity may not preclude proceedings against Israeli officials because of Israeli inaction, Hamasnikim are not subject to anything that remotely resembles a judicial system worth the name, so there is nothing comparable to even fail to act.)