I've written on the Anglo-Dutch union proposals of 1651-2, in my 1995 book on Oliver Cromwell's foreign policy - for which I dug out some of the original documents n the UK public Records Office in London on the English negotiators' position and ideas on what they wanted. At this point the republican 'Commonwealth' government in London, an uneasy coalition of those MPs, civilian merchants and gentry, and New Model Army generals among the Parliamentarian faction (civil-war-winners in England in 1646) who had been prepared to go along with the NMA's purge of moderate MPs in Dec 1648 and execution of Charles I in Jan 1649, was also in military control of Scotland and most of Ireland, effectively creating a proto-'United Kingdom' (or rather republic) style state. Scotland, then run by the English regime's fellow-Protestants but still recognising the Stuarts as kings under Charles II, had been militarily occupied in 1650-1 (and Charles II had just lost the battle of Worcester and fled to France), and Ireland, mostly run by anti-Commonwealth Catholic hardliners under the Confederation of Kilkenny, had largely been occupied too - by Cromwell and the NMA with a massive eviction of Catholic landowners in favour of incoming Protestants. The shaky English/ UK regime was feared by the Catholic monarchies of Europe, led by Habsburg Spain (allied to the Holy Roman Empire) and its rival the Bourbons of France, as an expansionist regicide Republic - and the Dutch, rebels against Spain who had had their long struggle for independence since the 1580s recognised by Spain at the Treaty of Westphalia in 1648, were Protestant, commercially orientated, and a major naval power so were seen in London as their 'natural' allies. Hence the moves for an Anglo-Dutch close alliance and even some form of union - which would put commercial rivalry and rival imperial claims in the East , eg over the 'Spice Islands' (Indonesia) and access to trade with Mughal India, aside in favour of religio-political solidarity.
Even at this stage there were rival Anglo-Dutch claims to fisheries in the North Sea and 'Cod War' style naval confrontations - things don't change in fishery disputes over 370 years - and also the English navy and ultra-patriotic English MPs were insisting on the prior rights of England to control its local waters and force all passing Dutch ships to dip their flags to English vessels in submission or be shot at. The merchant MPs in London had also passed the Navigation Acts which had demanded that all goods shipped in and out of England be moved in English ships to aid their shipping industry and to ban overseas nation's ships from UK harbours, as referred to in the article. It was a matter of whether the questions of commercial rivalry or politico-military alliance would come first in the talks, and the weight of pressure for a hard line was backed in London by most of the House of Commons - dismaying Protestant zealots like Cromwell who put a anti-Catholic alliance first. Neither the English nor the Dutch would back down, the inevitable clashes at sea were played up by both sides and reparations for losses and naval retaliation demanded, and the situation escalated to war in 1652. Some of Cromwell's militant Prot allies were so angry at greed and commerce defeating 'holy' religious solidarity that they encouraged soldiers and sailors to refuse to fight - and when Cromwell took power by evicting the Commons and set up his own govt in April 1653 he made peace-talks a priority. But neither side would give much away and the war stumbled on until English naval victories and the threat of them blockading the Dutch coast and starving the Dutch out (the latter had to import most of their food) drove the Dutch into a treaty in early 1654.
The question of a 'union' of the two states, with mutual citizenship and residence rights, was raised by Cromwell's allies (St John the Eng ambassador in 1652 was his cousin and a former close political mentor to him, now uneasy at having a republic, and the other UK ambassador Walter Strickland was another friend who sat on his Council in 1653-8 as its diplomatic expert) . It was then sidelined as the majority of the Commons wanted commercial matters to take priority and the Dutch to give way totally, eg on the Navigation Acts, and no treaty that would water this down would pass the Commons. After April 1653 Cromwell had no Commons to keep happy, but many of his allies were still too aggressive about the required terms to accept anything short of a Dutch climbdown and an English-led not equal federation; the zealots argued that God had shown His support for the Commonwealth by helping them to defeat Scots, Irish and Dutch so the latter had to submit to His will and accept English leadership. Some of the Cromwellians did however plan to offer the Dutch a treaty that would sort out commercial/imperial rivalry by dividing the world into separate spheres of interest - the Dutch would get Asia where they already dominated trade and the English would get the Americas. The two navies would then cooperate to kick the Catholic powers , esp Spain in S and Central America,out of their colonies and to annex these. (This was the 'Vermuyden Plan', thought up by the Dutch engineer Cornelius V who was a resident of England and was draining the Fens in E Anglia.)
The scheme was too idealistic and affected too many vested interests to be accepted -and in both 1651-2 and 1654 the Dutch commercial elite, led by the Amsterdam merchants, and their ruling republican legislature (the States-General, representing 7 Dutch provinces) and executive (led by Grand Pensionary Jan De Witt) were too suspicious of the English to want any close union . Also, some of the 7 provinces were hostile to the English republic as they were pro-Stuart landed gentry and supporters of the currently deposed Dutch ruling Orange dynasty , whose late head William II (W III's father) had been married to Charles I's daughter - and they would veto any treaty that gave too much away. The plan was a non-starter and showed English republican /Prot imperialist naivity , though Cromwell and his friends did have a few useful ideas on how to run it fairly with a governing executive council representing both sides, on the basis of Ancient Greek federal councils, and a limited degreeof powers for this body that left most domestic matters to a lower level of autonomy. Interestingly, the Dutch condemned the union as being too like the current 1652 creation of an Anglo-Scots union - ie the English had the bigger army and resources and the large number of delegates in the deciding federal Parliament in London and were doing what they liked, using the Scots as puppets. The same issues in fearing 'unions' in the 1650s as the 2020s?