The bill as currently written can probably be circumvented in such ways as you propose; however, the bill is still powerful for several reasons. First off, if it passes, it would politically demonstrate the unity of a majority of the House of Commons for an extension.
In the past, when the House of Commons has clearly demonstrated a majority opinion, the Prime Minister (PM) has tended to honor such opinion. This is illustrated most clearly by the 13 March 2019 vote of the Commons to reject No Deal, the 14 March vote to support some kind of extension, and the PM's subsequent honoring of those votes by requesting an extension at the EU Council on 21 March.
Moreover, if the House of Commons demonstrated such a majority opinion, it would influence the subsequent willingness of the EU Council to accept a requested extension at their meeting on 10 April.
Furthermore, the bill allows two opportunities for the extension date proposed by the government to be changed: Subsection 1.5 allows the opportunity for the House of Commons to change the date by amending the motion at that time. Subsection 1.6 and 1.7 allow that if the EU Council rejects the proposed date and counteroffers a different date, then parliament would have the opportunity to accept the EU Council's counteroffered date.